One of the many features of the 2BA-221 is the multitude of inputs and outputs. Not only does this make it easy to install it into your studio, it also gives you tons of flexibility when it comes to tracking.
Let's start with the outputs. The 2BA-221 has a total of 4 outputs. The main output is of course the +4dB XLR Tube output. Right next to that is the -10dB 1/4" TRS Tube output. To the left of the tube outputs is something called the "stacking" output; this is the solid state output also running at -10dB. This is important to note: the polarity on this output is inverted in relation to the other outputs. Where is the forth output? The INSERT jack can also function as an output by simply plugging in an unbalanced (TS) 1/4" jack. This is solid state output number 2.
Now it's time to multi-track a single source. Let's say we're tracking a vocal. Plug in your choice of microphone into the mic pre input. Set the solid state gain to a good level by watching the three segment LED meter, adjust the impedance, HPF, and polarity switch where you think it's best (you may want to change these later of course). Now you have the opportunity to run all these outputs to different sources and track them different ways. For example:
XLR +4dB tube output -> compressor -> recording device
1/4" -10dB tube output -> recording device as an unaffected backup
1/4" solid state stacking output -> multi FX box -> recording device
1/4" insert output -> distortion pedal -> recording device
Arm all 4 tracks and record your vocal part. You now have four copies of that vocal part to choose from and mix together, all with a safety back up in case anything went wrong with the others. That -10dB tube output safety backup is a great idea when recording; you never have to worry about missing a great performance because of overloading your recorder's inputs. Just keep running the lower level output into it's own track and you'll always have a backup.