Ok, this is a bit confusing, but bear with me.
Balch is 'U' shaped- there are 4 units overall. If you're facing the front, standing beneath the arch at the bottom of the 'U', the two units on your left are Units 1 and 2. The two on your right (which also have the Carol Tatkon Center and Balch's Womyn's Center) are Units 3 and 4. There are 5 floors in Balch, floors 2-6. Not each unit will have rooms on each floor (ie. Unit 1 does not have a 6th floor, Unit 4 does not have a 2nd floor because of the Tatkon).
So, for example, my room from last year was 4276 (a fantastic room

). The '4' indicates the floor, 4th floor. The '2' indicates Unit 2, the left side of the 'U'. '76' is the room #.
Floors are grouped by 'unit'. I was in 4-2. Each unit/2 units has its own RA, and its own toilet. A few units are split by a few stairs, as was my unit- there was a 'lower' and an 'upper' 4-2.
Each section has its own lounge. Unit 1 has a BEAUTIFUL study lounge, Unit 2 has a TV lounge and a study lounge, Unit 3 has a TV lounge, and Unit 4 has a small TV lounge and a study lounge. You can obviously use any of the lounges. A nice benefit is also the fact that you can walk to the Tatkon Center and get a smoothie in your slippers!
I hope this was helpful/easy to understand.. if you need more clarification just let me know!