I need to some restriction in fields ( Actual GI Date, T-Code:Vl01n ).
How do you find out whcih user exits belongs to VL01n ?

Here is the list of user exit related to VL01N :

V02V0001   - Sales area determination for stock transport order 
V02V0002   - User exit for storage location determination 
V02V0003   - User exit for gate + matl staging area determination 
V02V0004   - User Exit for Staging Area Determination (Item) 
V50PSTAT  - Delivery: Item Status Calculation 
V50Q0001   - Delivery Monitor: User Exits for Filling Display Fields
V50R0001    -  Collective processing for delivery creation 
V50R0002    - Collective processing for delivery creation 
V50R0004    - Calculation of Stock for POs for Shipping Due Date List
V50S0001    - User Exits for Delivery Processing 
V53C0001    - Rough workload calculation in time per item 
V53C0002    - W&S: RWE enhancement - shipping material type/time slot
V53W0001   - User exits for creating picking waves 
VMDE0001  - Shipping Interface: Error Handling - Inbound IDoc 
VMDE0002  - Shipping Interface: Message PICKSD (Picking, Outbound) 
VMDE0003  - Shipping Interface: Message SDPICK (Picking, Inbound) 
VMDE0004  - Shipping Interface: Message SDPACK (Packing, Inbound) 

By : Nimesh Jhanwar

Fast Links:
How To Find User Exit
Finding the user-exits of a SAP transaction code

Understanding Field Exits
Field exits (SMOD/CMOD) Questions and Answers

Get help for your ABAP problems
Do you have a ABAP Question?

SAP Books
SAP Certification, Functional, Basis Administration and ABAP Programming Reference Books

ABAP Tips
ABAP Forum for Discussion and Samples Program Codes for Abapers

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.sap-img.com