Here's the IPC Commentary concerning "service sinks":
The “service sinks” required by Table 403.1 are intended to be of a type suitable for janitorial and building maintenance purposes. Service sinks include mop sinks/basins, utility tub/sinks, janitor sinks, slop sinks and laundry trays. Only one service sink is required for the entire building, except that hospitals are required to have a service sink on each floor. The one service sink per building should be in a location that can be accessed from all portions of the building. For example, in a building that has multiple tenant spaces, locating the service sink for the entire building in a tenant space that cannot be accessed by janitors for a different tenant would be inappropriate. Persons responsible for janitorial and maintenance services in the building have access to the required sink without the need to enter one tenant space in order to service a different tenant space. This does not mean that the service sink must be in an open area of the building for use by any occupant, such as visitors. The service sink can be located in a locked janitor’s closet to which the appropriate personnel has access by key or door lock code. Note e to the table allows business and mercantile occupancies of 15 or fewer persons to not require a service sink. Service sinks in these small occupancies are rarely used.
