Syntax
Snippet taken from /usr/share/X11/xkb/symbols/latin:
key <AE01> { [ 1, exclam, onesuperior, exclamdown ] };
key <AE02> { [ 2, at, twosuperior, oneeighth ] };
key <AE03> { [ 3, numbersign, threesuperior, sterling ] };
key <AE04> { [ 4, dollar, onequarter, dollar ] };
Each line defines a key.
<AE01> defines a key. The second letter defines the row where it is in (E = fifth row from bottom). The 01 says it's the first key from the left. (The special characters even further left apparently don't count)
So, the x would be identified on some western keyboards with <AB02>.
Function keys are identified with FK..
After identifying the key, there are four mappings. The first is for pressing just the key, the second for combining it with the shift key, the third for combining it with the right alt key (altgr) and the fourth for combining it with shift+altgr.