MPPT load terminal

My guess is that they really mean "High power battery chargers." In other words, something that might draw a great deal of current.  According to the manual, the other things they specifically mention not to connect to the load terminals are "Inverters" or "High Amp Drawing Device."

Also, I'm sure they contemplated that some...intellectually challenged...individual might try to charge his deep cycle marine batteries by connecting a charger to the load terminals (even though they're connected to the controller!).  Putting a prohibitive note in the manual banning chargers on the load terminals might prevent that.  Then again, I'm sure there has to be a person out there who thinks they can connect a 9V transistor radio battery to the controller.  I was an electronics repair/calibration technician for many years...and I've seen it all.

The other possibility is that they don't want something connected which could possibly backfeed voltage/current back into the controller's load circuits when the load feed is off.  Some cheap chargers can do this.

In my opinion, charging a cell phone, tablet, etc. from the load terminals through a 12VDC outlet with 5VDC USB converters shouldn't be a problem.  But you may want to give the Renogy techs a call to confirm this just in case.  If you do, be sure to report back with what they recommend.