For this question, advise the Jeppesen GSPRM 2017 - Introduction Section - (GSP) ENROUTE-6, 7, 8 pages, where a sample of an Enroute chart is given with symbol explanations aside.

• When an airway is marked as 3500T, it means that its Minimum Obstacle Clearance Altitude (MOCA) is 3500 ft.
• When an airway is marked as 2100a, it means that its Route Minimum Off-Route Altitude (Route MORA) is 2 100 ft.

MOCA is the lowest published altitude in effect between radio fixes on VOR airways, off-airway routes or route segments, which meets obstacle clearance requirements for the entire route segment.

Route MORA is an altitude which provides reference point clearance within 10 NM of the route centreline and end fixes. Route MORA values clear all reference points by 1000 ft in areas where the highest reference points are 5000 ft MSL or lower and by 2 000 ft where the highest reference points are 5001 ft MSL or higher.