Lets look at this example. Some value of a dimension (say d1) needs to be either 0 or 10. You can write the following relations:
/***************************
VALUE=d1-5
IF VALUE <5
d1=0
ELSE
d1=10
ENDIF
/****************************
The parameter VLAUE can be defined only in the relations if needed. I am not sure if this answers your questions, but it can be abuilding block of what you want.