As noted, there are three, main operating frequencies: 15, 30, and 80 kHz. It’s borderline false advertising to say the ORX has 21 frequencies as the all the additional frequencies from the main three are actually frequency shifts. Frequency shift is used for slightly changing the operating frequency to minimize EMI or interference from nearby detectors operating at the same frequency.
This is hardly incorporated by most users, but the feature is there for convenience regardless. With frequency shift, you can slightly adjust each main frequency from 13.3-15.6 kHz, 26.6-31.3 kHz, and 68.3-80.8 kHz.