What do you mean my a Task and where do you assign and How?
Task is a description of an activity in Organizational Management which is performed within an organizational unit. 
one can assign an organizational unit, job, position, person (employee) or user a task in order to describe their function in an enterprise. A task which you assign, represents a characteristic of an object.

What is an Org's assignment and Org'l Plan, what is the difference between them?
Plan versions are scenarios in which one creates organizational plans. 
In the plan version which you have flagged as the active plan version, you create your current valid organizational plan. This is also the integration plan version which will be used if integration with Personnel Administration is active. 
As a rule, a plan version contains one organizational structure, that is, one root organizational unit. It is, however, possible to create more than one root organizational unit, that is more than one organizational structure in a plan version. Organizational assignment is assigning the persons i.e., employees to certain position according to organizational structure and plan.

How many structures are there in OM?
In the SAP System one can use several structures to represent the legal and organizational structure of your company. You can structure from the point of view of accounting, materials management, and sales and distribution. It is possible to combine these structures. The organizational structures form a framework in which all business transactions can be processed

What do you mean by reporting Structure?
Reporting structure is the organized hierarchical structure where the different positions (persons) will have to report to various positions according to your organizational structure.

What is the difff between Simple Maintenance and Detailed Maintenance? and which one you used in your project?
Simple Maintenance in Expert Mode is one of the methods you can use to develop and model organizational plans in the component Organizational Management with speed and efficiency. Besides mapping your current organizational and reporting structure, Simple Maintenance gives you flexible tools to plan and model future structural changes well in advance. SAP Business Workflow users do not need all the functions available in Organizational Management. For this reason, the original concept behind Simple Maintenance was to provide a tool that allows users to build and maintain organizational plans, with speed and simplicity. For Organizational Management users, Simple Maintenance is best used to establish the basic framework in organizational plan development.

Simple Maintenance uses a tree structure, which allows you to create a basic framework for organizational plans, using streamlined procedures.  In this way, one can create ones organizational and reporting structures step by step.

What are the building blocks in OM and where do you assign them?
Organizational units, Positions Tasks, Persons , etc, are the building blocks of O&M. you create them as part of your organizational plan.

What do you mean by work centre and Cost center? Where do you assign them. And how many Cost centres can we assign?

Cost centre is an object used to refer to work centre activities are going on from cost point of view which is important from cost point of view. Work centers are the objects (physical locations) where actual work takes place

How do you transfer Org'l Assignment from PA to PD? (Answer for this is a bit diff, try it)  
I think this can be done by editing your staff assignment. Here you can decide which positions belong to the organizational unit.

What are the info types you used in OM?
From 1000 to 1999       *-- Madhuuri

What are the infotypes used for time management?
2001 absenses
2002 attendances
2003 substitutes
2004 availability
2005 overtime
2006 absence quota
2007 attendance quota
2010 employee remuneration information 
2011 time events
2012 time transfer specification
2013 quota correction
2050 annual calender
2051 monthly calender
2500 pesonnel work schedule time
2501 employee time and labour data      *-- Somasekhar

How do you create an evaluation path?
Evaluation paths are created in organizational management Customizing under basic settings. One or more relationships form the navigation paths for an evaluation. These enable us to report on and display structural information, for example, organizational structure or the reporting structure.  We can create alphanumeric evaluation paths with a maximum of 8 characters starting with Z.

What is an infotype?
Well an infotype in short is a "type of information". It is a set  of logical data grouped according to subject matter, eg. Addresses  (organization assignment, personal data, addresses, planned working  time, basic pay, bank details, etc.) infotype, contains name, first  name, Address for correspondance, Permanant address, city ......so on.  There are over 5000 infotypes exists.  The infotypes can be created, changed, copy, delimit, deleted..

We can create infotypes useing the following path:
  Org Mgt->expert mode->(Object) The Maintain(object) screen appears
  In plan version and object fields enter data as required
  In the infotype field , select the infotype you want to create
  Using the tab pages select the status the new infotype record
  Choose Edit->create infotype
  Enter appropriate fields and save....        *-- Souji

Explain the uses of the simple maintenance interface?
Simple Maintenance is used when Staff assignments and reporting structure are to be changed.  There are three main areas in Simple Maintenance.  Each area contains particular maintenance functions, depending on whether you want to edit organizational structure, staff assignments or task profiles.    For Organizational Management users, Simple Maintenance is best used to establish the basic framework in organizational plan development.    For complete, detailed editing of individual organizational objects in your organizational plan (editing particular positions or organizational units, for example), we recommend that you switch to Info-type Maintenance.    Simple maintenance uses a tree structure, which allows you to create a basic framework for organizational plans, using streamlined procedures.  In this way, we can create an organizational and reporting structures step by step.

What is the use of INFOGROUPS?
The infotype group (commonly known as infogroup) guarantees that during the personnel action, all information needed for the business processes is stored.   The infogroup is processed when you perform a personnel action.
-  An infogroup exists in the standard system for every personnel action type.
-  In Customizing for Personnel Administration, you can tailor the make-up of the various infogroups to the requirements of your company.
-  You can define infogroups as user-group dependent.   *-- Debjani Paul

Infogroups are used in Personnel Actions to define a set of sequencial infotypes that you would be using while performing an action. For example, when you hire, you would want to use IT0000,0001,0002,0006,0007,0008 etc in a sequence- in a particular order. So define an Infogroup for this purpose which contains all these infotypes, their order of execution and the user groups it is applicable for. Then you assign this infogroup to the action that you had defined.  *--  Krish

What is PROCESSING CLASS, where we do them and why?
Wage type characteristic that determines how processing is conducted during the payroll run.

What is EVALUTION PATH, where do we do it and why?
A chain of relationships between objects in a hierarchical structure.
for eg. O-S-P(Organization->Position->Person).

What is INTERNAL PAYROLL PROCESS?
Payroll process(program) is run at a specific point in time, not only to calculate an  employee’s basic remuneration but also any special payments, overtime payments or bonuses that must be effected for the period in question.   *-- Souji

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