when you are below FL180 you may get a clearance for a victor airway, the radials of an airway, or as mentioned a standard terminal arrival route(STAR) .
Let' say there is a jet airway you are cleared on and you are cleared to descend to 17,000'. while the jet airway no longer exists, you may be cleared VIA THE RADIALS OF JET AIRWAY you were still on. Or an underlying victor airway.
conversely you may be cleared via a victor airway and when at or above FL180 you may be cleared via an overlying jet air way or the radials of the victor airway.
it is a good technical point and if cleared via a J route below Fl180 or a victor airway above FL180, you should clarify it with ATC...they might chuckle and then clarify...but be sure YOU know which way you are going