NRPN
Non-Registered Parameter Number — MIDI's way of addressing more parameters than 128 controller numbers allow, by spreading address and value over several messages.
An NRPN change is a short run of Control Change messages: CC 99 and 98 select the parameter number, CC 6 and 38 carry the value. That yields 16,384 addresses at 14-bit resolution instead of plain CC's 7 bits. Its sibling RPN uses CC 101 and 100 for parameters defined by the standard itself — RPN 0, for instance, is pitch bend range.
It isn't graceful. Four messages per knob movement eat bandwidth on a 31.25 kbit/s cable, and if another CC sneaks in between, or the receiver powers up mid-sequence, the value lands on the wrong parameter. What NRPN 1234 means is up to each manufacturer. MIDI 2.0 turned both into single messages with 32-bit values.