YEAR vs &YEAR Tag differences in Tag Panel

Some of the TAGS have & Album and &Year and some are just Album Artist

What is the difference?

Check the documentation on customizing the tag panel:

and the use of the ampersand.

It just says - If you’re using the ampersand & character in the name of a tag field, the following character will be available keyboard shortcut in combination with the Alt key.

But that doesn't tell me what the difference is, it just says if you use it you can do something with the alt key.

What character, the ampersand? Why would I want & to be available?

Yes. That is what it does: it adds the character following the & as shotcut.
So, in your example tag panel you could press Alt-Y to jump directly to the field for Year.
Or Alt-i to go to Artist.
If you want to go to the field for Discnumber, though, you would either have to use the mouse or press the Tab key a couple of times until the keyboard focus has been moved to that field.

Just to confirm, the tag panel is the panel at the top left in the default view with title, artist etc in it.

As the reason I ask is alt-i gets me nowhere, none of the shortcuts seem to work in windows 11.

But many thanks for clarifying!

Apparently, the function is case-sensitve.
And the keyboard focus has to be in the tag panel.