Here is an example of using :$FLEX$.Value_Set_Name to set up value sets where one segment depends on a prior segment that itself depends on a prior segment ("cascading dependencies"). Assume you have a three-segment flexfield where the first segment is car manufacturer, the second segment is car model, and the third segment is car color. You could limit your third segment's values to only include car colors that are available for the car specified in the first two segments. Your three value sets might be defined as follows:

Segment Name Manufacturer  
Value Set Name Car_Maker_Name_Value_Set  
Validation Table CAR_MAKERS  
Value Column MANUFACTURER_NAME  
Description Column MANUFACTURER_DESCRIPTION  
Hidden ID Column MANUFACTURER_ID  
SQL Where Clause (none) 

Segment Name Model  
Value Set Name Car_Model_Name_Value_Set  
Validation Table CAR_MODELS  
Value Column MODEL_NAME  
Description Column MODEL_DESCRIPTION  
Hidden ID Column MODEL_ID  
SQL Where Clause WHERE MANUFACTURER_ID = 
:$FLEX$.Car_Maker_Name_Value_Set 

Segment Name Color  
Value Set Name Car_Color_Name_Value_Set  
Validation Table CAR_COLORS  
Value Column COLOR_NAME  
Description Column COLOR_DESCRIPTION  
Hidden ID Column COLOR_ID  
SQL Where Clause WHERE MANUFACTURER_ID = 
:$FLEX$.Car_Maker_Name_Value_Set AND 
MODEL_ID = :$FLEX$.Car_Model_Name_Value_Set 

In this example, MANUFACTURER_ID is the hidden ID column and MANUFACTURER_NAME is the value column of the Car_Maker_Name_Value_Set value set. The Model segment uses the hidden ID column of the previous value set, Car_Maker_Name_Value_Set, to compare against its WHERE clause. The end user never sees the hidden ID value for this example.