Summary

This article explains how to create a single Property who's value is different depending on which release flags are chosen at build time.  For example if a project has 2 different releases which each use a different release flag: 

Example: 
Release 1 
flags _FLAG1_ 

Release 2 
flags _FLAG2_ 

Property 
MYPROPERTY 

Value to be set (by _FLAG1_) MYPROPERTY = "Value1" 
Value to be set (by _FLAG2_) MYPROPERTY = "Value2" 

Discussion

This can be achieved by creating 2 "New Set Property" custom actions which create the same Property but have different conditions:

Custom action 1: 
Property Name: MYPROPERTY 
Property Value: Value1 
Add a condition of: ISReleaseFlags><"FLAG1" 

Custom action 2: 
Property Name: MYPROPERTY 
Property Value: Value2 
Add of Condition of: ISReleaseFlags><"FLAG2"