Confusion interpreting an amplifier module's cut off frequency from its datasheet
1) It's not clear what 'changeover' means here, it's probably a bad translation from another language. Fortunately, 3rd order Bessel lowpass filter is unambiguous. Bessel means a lowpass filter response which is free of time domain overshoots, ideal for measurement applications. If you put a step change into it, the output will rise monotonically to the final value, sharper filters like Butterworth or Chebychev would overshoot the final value before settling down.

Changeover might refer to the change between passing low frequency signals and blocking high frequency ones, or it might refer to the switch being able to change the response between 10Hz and 6kHz, who knows?

2) Normally, cutoff is defined as whatever it's defined as. If there's no figure supplied, then it often means the -3dB point. Here, it is defined, and as the -1dB point. 

What's being defined is the flatness of the passband up to the specified frequency. It's quite common in measurement applications to specify a flatter passband than for (say) audio. -1dB means about 10% error, whereas -3dB would be about 30% error.

Given that we're told the shape of the filter, we can use standard curves to estimate the -3dB point from the -1dB point. For -1dB at 10Hz, a 3rd order Bessel will be -3dB at roughly 16Hz.