The CPU Usage Limit value will allow up to 85% of the total CPU time to be devoted to running RTAS plug-ins, with only 15% left over. When using less than the Max allowed, Pro Tools uses whatever it needs. If you want to make sure that say, at least 35% of the CPU is always available for things like responsive meters/graphics, plug-in automation, MIDI,...then set it for 65% so you'll get -9128 errors when you hit the 65% limit. If you aren't using many RTAS plug-ins, you'll probably never hit the limit (but you'll know when you get a -9128 error).