User exit in Validation in Project Sysyem.

Dear All,

During creation of Project Profile we assign the validation rule in the Proj Profile.

The validation rules defined have a structure- Pre- requisite>check>Message.

Once the Prerequisite is met, there is a check

The check could be a boolean statement or a exit-Uxxx

Assuming the Exit has a code/logic to check the requirement...

<b>Question is How the Exit is defined and designed and associated to the exit line items in the validation.?..

How can we add/modify the exits.?...</b>

Thanks in Advance..