Use and Meaning of Capacity: Capacity Dropped below 15 %

I imagine that you have seen dictionary definitions supporting your interpretation of capacity as the maximum that can be held.

For example,

Capacity = the total amount that can be contained or produced

Cambridge dictionary

The use of the word in your sentence is a shortened form of “residual capacity”, the amount of the maximum that remains to hold new admissions to ICUs. If 85% of the (maximum) capacity is already in use, the residual is 15%. Hence we may understand that in some regions the amount of (maximum) capacity already in use rose above 85%, thus reducing (residual) capacity to less than 15%.

Or it may be similarly interpreted to mean the capacity available to accept new admissions to the ICU. As patient numbers in the unit increase, the available capacity decreases.

I am so glad you asked this. I have read at least half a dozen news outlets make this mistake in California in the last week.

I just read one that said "ICU capacity has dwindled across the state," which is totally wrong. That would mean the total number of Intensive Care Units, both in use and out of use, has dwindled.

Capacity means 100% of something. You could say 85% of capacity and it would be correct. Or you could say only 15% of capacity is left/available.

Funny thing is that even though it is wrong it is not getting caught by the news editors because everyone who reads it still knows what is meant.