Fixed costs are identified with those which do not increase with output and variable costs with those which do increase with output. This is clear for instance in manufaturing (raw material, distribution, direct labour,... are variable costs and real estate, marketing, ... are fixed costs).
However, for service business this distinction is not that clear to me. For instance, in the case of a bank, adding one more customer (and hence increasing the revenue from fees or interests) does not necessarily increase the number of employees or offices to be handled; considering this labour and real estate could be defined as fixed costs. However, if the number of customers grows significantly the bank will need to increase its labour force and probably also the number of offices. So how would you establish the boundary between fixed and variable costs in these type of cases?