The CPU Usage Limit value is the threshold that Pro Tools uses to determine when to report a -9128 "too many RTAS plug-ins using too much CPU" error. In other words, the lower this value, the sooner these errors start happening. If you are using RTAS DirectConnect, then raising this number would allow more DirectConnect (or other RTAS plug-ins) to run before hitting the threshold. This setting doesn't affect TDM DirectConnect, so it wouldn't make much difference until you started running out of processing power either with too many RTAS plug-ins or too many host synth/sampler apps running. Then you'll either see -9128 errors if it is set low enough, or you'll get underflow problems with the DirectConnect/host synth/sampler signal path.