Jeppesen Low Altitude Enroute Charts

[Heliplane]

I recently picked up a Jeppesen Low Altitude Enroute Chart to keep in my flight bag in case of emergencies (I bought this by itself and thus do not have the overall Jeppesen guide).

The minimum enroute altitudes for airways segments are mostly specified as flight levels - simple enough - but on certain segments, the letters "T", "a" or "g" follow the relevant altitude (eg 7000T, 6000a).

I recall that the "T" suffix denotes the minimum obstacle clearance altitude but can someone let me know what the others mean?

I've scoured these forums and the internet for a guide to these Jeppesen charts but can't seem to find one. Unfortunately there is no legend on the charts themselves. Does anyone know if one is available?

Thanks!