800
400
800A frame size, 400A trip unit
This means that the FRAME of the breaker will ACCEPT up to an 800A trip unit, so the POWER COMPONENTS can handle up to 800A, but the unit in question currently has a 400A trip unit (or plug, or CT etc. etc.) so it is intended to be used only on a 400A circuit. This is done so that if someday in the future the circuit size needs to increase, all you have to do is change the trip unit, assuming of course that the cables are properly sized (which is often done with future consideration issues).
If you had a 400/400, then there is no chance of some day increasing the circuit size without changing the entire breaker. If in a switchboard, that may end up entailing a rearrangement of several breakers inside to make physical room for the larger frame. This all increases down time. In draw-out gear that is not usually an issue, so often it is based simply on initial cost. If someday you want to increase the size and you started with too small of a frame, a replacement cubicle is a LOT more expensive than buying an oversized one in the first place and changing the trip unit/rating plug