That is really undocumented (as almost everything with the cubes). As far as I could understand it that is an acronym for the link path for the field, T standing for Task and R for Resource. Recall that assignment is a relation between tasks and resources. This only happens in the assignment cube.
I've used any of them randomly with no trouble, so I believe you can just ignore it.
tozevv
2009-12-10 18:10:30