They accuse you of being in a bus lane.
This (https://tro.trafficpenaltytribunal.gov.uk/TRO/Sheffield/FD01B.pdf) is the 1991 parent order that regulates their bus lanes. Note that the definition of bus lane refers to areas of carriageway listed in schedule 1 and 2.
This (https://tro.trafficpenaltytribunal.gov.uk/TRO/Sheffield/BP-Consolidation-1991-Amd-80.pdf) order amends the 1991 parent order. Note that it adds the Arundel Gate bus gate to schedule 3 of the 1991 parent order. According to their own definitions, Arundel Gate bus gate is not a “bus lane” since it is not an area of carriageway listed in either schedule 1 or 2 of the 1991 parent order and so the alleged contraventions did not occur.