Lorne's reference to compresor sweating is the condensing of moisture on the compressor while in the cooling mode. This would indicate insufficient superheat in the vapor. (If you set the glass filled with ice on the table in the summer, it "sweats.")
As to your question; the allowable percentage of airfliow reduction will depend on how far off the airflow is already. For the sake of explanation, let's assume your airflow is already perfect. So, a three ton unit moving 400CFM/ton needs 1200CFM. Anything below 300CFM will cause icing and prompt compressor damage, so we can use the simple assessment of 25% reduction in actual airflow to be the maximum.
But; as you close off vents, your static pressure increases, causing more air to come out the remaining open area. Up to a point. The ducts have a maximum capacity for airflow at the unit's maximum capacity for creating static pressure. I know that is a confusing sentence, and a confusing concept, so let's reassess the "perfect" airflow situation.
You still have a three ton unit, moving 1200 CFM, so it's still perfect, right? But, the ductwork is a little undersized, and the unit is generating .5 inches of water column in static pressure. In a situation like that, when you close off 25% of the vents near the unit, you have actually cut off 1/2 the airflow.
But, if your ducts are designed especially with zoning in mind, and they provide an even distributed 1200 CFM at .01 inches of water column external static pressure, you could conceivable close off 75% of the open grill area without reducing the airflow 25%.
I hope this makes sense, because it is the combination of several concepts not previously discussed in this context. But, this is what Lorne has been trying to say.
There are no stupid questions. . .just a lot of inquisitive idiots.