The track is set up so that a single curtain is pulled from either the left or the right hand side (you specify). The motor side will always be on the side the curtain stacks. Ideal for asymmetric windows where there is less space to stack the curtain on one side than the other.
This is the standard operation where two curtains on the same track pull from left and right to the centre. Classic look for elegant rooms. Track motor can be fitted either side, whichever is convenient for the power supply.