Is there a standard stating how fast a fast fuse is, how slow a slow fuse is?
Individual manufacturers may have a consistent naming systems to identify slow, really slow, fast, turbo fast, or whatever, within their product line.  Some smaller manufacturers may have copied parts of the designations from a dominant manufacturer, but you shouldn't count on that.  Ultimately, only the datasheet tells you how fast or slow fast or slow really are.

End users shouldn't have to consult datasheets, and can't be counted onto understand them if they did.  This is no different from any other component in a device.  The way to handle this is to tell them the replacement part number explicitly.

For a common type of fuse, you can write something like "1A 250VAC" near the fuse holder, but that should be for convenience at best.  Somewhere in your documentation it should say that the fuse is a Acme ICUB4UCME-1A, or equivalent.  If they put in something else, the liability is now on them.