What kind of nut is this
I find using the correct “c” spanner, in fact I would use two together to undo that as it means no hammering to possibly damage bearings etc.

This method of using two nuts is used to set a limit or stop position - if only one nut was used it could easily move. A "nyloc" nut may also move but the two nuts locked together do not move. Some nuts can have a set-screw inserted to prevent movement but that is less secure than the two nuts...

Called a c spanner because it looks like a c but has a tooth at the end to engage in the slot on the nut.