What do you call "internal oscillator"
Is it an oscillator built in the SF3301, or does it refer to the on board oscillator?
Is one on the PCB not visible on the right of their board picture?
yes, and it's also on the schematics, but even if you have an internal oscillator, It's quite common to use an external one for some reasons, so the "internal" word may be a translation mistake referring to the one on board as opposed to the one from another daisy chained board, the signal being transferred by the coaxial cable (RF receptacle).