For me, my X is mainly a mixture of objects or effects in my mp, eg weather , pet , environmental attribute: grass land … etc
Note I am still polishing my skill in PA for speed games. For long term memory I use PAO. PAOX I will hold on until I can master PAO smoothly.
To expand a system there you have two obvious ways, one is by expanding vertically single digits to 2-digits to n digits system. Or horizontally, PAOXXXX… X can be any extra attributes.