The short answer: Most commercial occupancies are required to provide at least one service sink — commonly a floor-mounted mop sink — under the plumbing fixture tables (IBC Table 2902.1 / IPC Table 403.1). It's there so the building can be cleaned and mops/buckets filled and emptied without using food-prep or restroom fixtures. A service sink can be a floor-set mop basin, a wall-mounted service sink, or a janitor's sink; it needs hot and cold water and a drain, and it's typically located in a janitor/custodial space. Though it's a single small fixture, it's required and frequently forgotten on plans — omitting the service sink is one of the most common (and easily avoided) fixture comments.
Why a service sink is required
The fixture tables require a service sink for most occupancies so maintenance has a dedicated point to fill/empty mop buckets and dispose of cleaning water — keeping that activity out of kitchen sinks, hand sinks, and toilets for sanitation reasons.
What counts
A floor-mounted mop basin (the most common).
A wall-hung service sink.
A janitor's sink.
Each provides hot and cold water and a drain, with a hose/faucet suited to bucket filling.
Where it goes
Typically in a janitor's closet or custodial/maintenance space, located for practical building cleaning. It should be shown on the plan with its connections.
The "one per occupancy" nuance
The requirement is generally at least one per occupancy/tenant (more for large or multi-floor buildings per the table). A multi-tenant building may need one per tenant space depending on the arrangement.
Common review comments
No service sink shown at all (the classic miss).
Service sink counted but not actually provided with hot/cold water and drain.
One service sink where the building size/configuration requires more.