Grasses
T -  Tall grass
M -  Mid-tall grass
S -  Short grass
B -  Bunchgrass
R -  Rhizomatous

Legumes, shrubs and forbs
E -      Erect, multi-stem crown
D -     Decumbent or prostrate (low growing or have a tendency to lie along the ground unless supported by surrounding vegetation.)
R -      Rhizomatous
S -      Stoloniferous

Season
C -   Cool season
W - Warm season

The terms tall, mid-tall and short are quite widely used, but have no well established definitions.  From Plant Ecology, Weaver and Clements, it can be inferred that they describe the mature plant complete with seed head.  In this stage short grasses will generally be under 2 feet, mid-tall grasses 2 to 4 feet and tall grasses greater than 4 feet.