I recently received my Re:Load Pro, and I'm looking forward to start testing some batteries with it.

I'm curious what the blue banana jack is intended to be used for. In the schematic I guess it would be J4, connected to pin 10 through a 100Ω resistor. Is it a remote (low) voltage sense? Remote thermistor connection? What?

It's an 'aux' connector reserved for future firmware support. Presently it's unused, but future uses include synchronization between multiple Re:load Pros.