What does the prefix "+/-" indicate before the data line names?

The +/- means that the transmitted and received signals are 'bipolar' relative to ground.
1 / idle / spacing / logical 1 is usually negative relative to ground.
0 / active / logical 0 is usually positive relative to ground.

Where a polarity is not specified (eg RS422) all signal levels are always positive relative to ground - but in a balanced signalling system such as RS422 the levels on the two data leads will be of opposite level t0 each other (1-0 or 0-1). .




Two signalling lines in a balanced pair.
"One goes up and the other goes down."
Both are usually within a single set of supply rails (eg V+ and ground BUT the signal is not ground referenced. The common mode voltage MAY need to remain within the rails or some limit BUT this is not part of the signalling system per se.
Example - RS422


The signal is on a single line referenced to (usually) ground.
It transitions above and below ground to signal (sending or receiving).
It is not balanced, is not inherently noise immune.
Example - R232.