“robotaxi” is a generic term for (when the term was coined, hypothetical) self-driving taxicabs, that predates Tesla existing. “Tesla Robotaxi” is the brand-name of a (slightly more than merely hypothetical, today) Tesla service (for which a trademark was denied by the US PTO because of genericness). Tesla Robotaxi, where it operates, provides robotaxis, but most robotaxis operating today are not provided by Tesla Robotaxi.