A Taboo (or No-Go) is a practice that a sex worker generally does not offer - regardless of the additional charge. Often explicitly listed on the profile or in the service catalog (e.g. "Taboos: AO, Caviar, GS active").
Taboos are binding. Attempts to negotiate or pressure to do so are unprofessional and, in the serious industry, a reason to end the appointment immediately - with full payment for the previous time.