What is the difference between room tone and ambiance?

Room tone is a particular type of ambiance, usually associated with a location where an interview or narration is recorded. Ambiance is a recording of the sound of a location, typically without conversation. Since the ambiant sounds may be quiet, normally you'd boost the recording volume to get the maximum signal to noise ratio. But room tone should be recorded at the same volume setting as the interview or narration done at that location and time. That way in the editing process small bits of room tone can be inserted to adjust pauses or provide sound at the beginning and end of an actuality, without audible level jumps.

Note that if you're taping room tone with a recorder that's set for automatic level control, it won't stay at the same level setting that was used for the interview or narration. The recording level will rise automatically to compensate for the relative quietness of the location without voices. This tape would be useable as an ambiant bed for mixing in, but strictly speaking it would not be room tone, as its levels wouldn't match the voice recordings.