Yes it is the system track on the 5000.
I use it to record my rhythms and the active registration (upper/lower part) at the same time. It is very nice but sometimes I need to replacr the drum part with my own user rhythm for example.
If you clear the accomp from the system track, you can enable overdubbing to keep the rest of the parts intact (u/l keyboard parts for example)
When you start to record, the keyboard part of the system track will playback and the new drums get recorded (overdubbed) on the previously cleared drum part.
This only applies for the system track.
Solo tracks are simpler as per Brad's comment above but I prefer to start with the system track (everything at the same time)