The default setting (+/- 180) means the framing process accepts either of the two angles – the angle entered or the angle entered +/- 180 degrees. When the mount pier flips, the camera rotates 180 degrees and the framing routine accepts the angle without further ado. The +/-180 setting gives the same behavior as before the option was added, so it is naturally the default.
Folks with rotators who want to use the same guide star after the meridian flip on a target would select “Always” to have SGP always rotate to the specified angle precisely. “Always” is the new setting, generally only useful for folks with camera rotators imaging in sparse star fields.