The b-th beat of the g-th beat group of the m-th measure, would unsurprisingly be Tmgb in this model.
It's ambiguous if I'm talking about absolute or relative here but it's Tmgb or tmgb respectively.
We'll call the level below the beat, the sub-beat and it's offset from the beat will be Tmgbs where s is the sub-beat number within the beat.
The sub-beat's offset from the beat will be tmgbs where s is the sub-beat number within the beat.