if you have source system > source location field.

double click source system > edit properties menu will display, click properties tab and click edit you will get query pane there you pull down source location object in condition pane and click inlist and click type new prompt type there "XXXXX" and save that.

now you click display button first source system lov will display and you select any one, based on that selection the source location lov will display.

this is cascading prompt