The Lisa Serial Port B is designed to switch between a single-ended RS232 configuration (as used by typical serial ports) and a differential RS422 configuration (as used by LocalTalk (originally called AppleTalk, but that name was repurposed to refer to a protocol layer vs the hardware layer).
The AM26LS30 line driver has a rare feature that performs this configuration change.
It was also used in the early Macintosh models for this same ability, but in the Macintosh, this was used for both serial ports so both could be switched to the differential mode (and both ports could be used for LocalTalk), whereas the Lisa's Serial A is single-ended only.
Note that the single-ended RS232 signals are still bipolar (switching between voltages above and below 0 V).