Spoon,
You are correct in what you say, but this is more of a description of effect rather than function.
The semi-closed units could best be described as "gas extenders". They feed a small amount of fresh gas from your cylinder either continuously, or when a mechanical control device senses loop volume depletion. They feed a little more than is needed, to provide a safety margin, and vent the excess overboard when necessary.
The fully closed units sense the oxygen percentage (in terms of partial pressure) in the gas you are breathing. They then inject just enough oxygen to make up for the amount your body uses. If you remain at one level in the water column, no gas will be vented because you get exactly what is required, and no more.
It must be noted that all rebreathers will vent gas on ascent due to volumetric expansion.
This is the quick and dirty explanation, but those are the basic differences. Hope that helps!