finish terminates an observation started by run and similar commands. The observation is saved to disk as if the full exposure-time had expired.
If the run was part of a multiple-exposure sequence (started by multrun etc.), finish only alters the current run. Other runs in the sequence carry on regardless.finish
An observation that was finished may be distinguished by the fact that its final, recorded exposure time is less than its demanded exposure time. Otherwise, there is no difference.
The finish command waits for the readout to finish.