Parking positions with an additional A or B should be used for all Medium Aircraft except the Boeing 757 whenever available (e.g. stand 01A, 02B). If using the stand without A or B you are blocking two stands and with the limited amount of parking positions available that reduces the apron capacity at the airport a lot.
Heavy Aircraft have to park at stands without any A or B (stand 01, 02, 06, etc.). If you are using A or B you are blocking 3 stands instead of just the A and B stands next to you.
To increase the apron capacity there are two additional lines (blue and orange) available for all aircraft with a wingspan of less than 36 m (A321/B737 family) to taxi next to each other. If you are unable to push onto such a line or use it, please report that to the controller as soon as possible!
Do not use the yellow line if you were instructed to use the orange or blue line!