A single-occupant toilet room: one WC (sometimes a urinal added), one lavatory, with a door that locks from inside for privacy. Because only one person uses it at a time, it serves everyone.
A single-user room isn't "men's" or "women's" — it's available to anyone. So in the fixture tally it counts toward the requirement for both sexes simultaneously, which is why a pair of single-user rooms can satisfy a small occupancy's requirement efficiently.
Where the required fixture count is small, the code generally permits single-user facilities instead of separate multi-fixture gendered rooms. The exact threshold is in Chapter 29 of the adopted edition. Larger occupant loads still require multi-fixture, separated facilities.
Newer editions and many state/local laws address all-gender facilities — some require single-user rooms to be signed as all-gender, some mandate all-gender options. This is an area of significant local variation, so confirm the jurisdiction's specific rule rather than assuming the model code.
Single-user/all-gender rooms need compliant signage and must meet accessibility clearances and fixtures like any required toilet room.
See also our fixture calculation guide and commercial restroom guide.
Model IBC framing; all-gender requirements vary widely by state/local law and edition. Verify against your codes of record and local amendments.