Sometimes a device will have a note like this “ Short-circuit rating achieved when used with respective fuse type and maximum fuse rating”
This is for an Eaton R9 type rotary disconnect switch. The 30A disconnect has a 100kA Short circuit rating with class J 30A fuses. Without it, you get the default rating for a switch which is 5 or 10 kA…
Ok - this makes the most sense so far. I hadn't been thinking about the SCCR of the disconnect itself - without the fuse.