How does "mechanical stop" and "electrical angle" differ for this potentiometer?
The electrical angle is the range of rotation over which rotation has an effect on the position of the potentiometer.  Outside that range, the wiper will probably be floating or in contact with one of the other terminals, and incremental changes won't affect the wiper voltage.

The mechanical angle is how far it can be turned overall.  This series may be limited to 340 +8/-0 degrees by a stop, or without the stop installed it can rotate continuously.