Mine is totally arbitrary, I stole 84 being my dog from Nelson Dellis but for most I used this ‘method’ I explain here:https://forum.artofmemory.com/t/pao-system-using-memes/103474/10?u=clarence
Edit: Thought I should explain this better, so from analysis, I do find that the method is like a category system, with the images depending on both digits. Like for most numbers ending with 4, they’re mostly boys and reasonably close to me, (I was 4 in my system before revising it, the vibe for 4 is nerdy, quiet boy in case you were wondering, now it’s Oliver Twist, I seemed not to be visualizing myself well enough). So 04 is a cousin, 14 was my brother (before revising, now it’s Charlie from Charlie and the chocolate factory), 24 is Harry Potter, but 34 is a girl for some reason, so both digits have a part to play. Like most 6_ are female, only 64 (Percy Jackson) and 67(Young Sheldon) are male among them. But in the end there isn’t much of a rigid rule set.
So that’s it
, for a lot of concepts and objects, I have a feeling association, though they aren’t always at the fore front of my mind. Acids are male, Bases are male too, Metals are male, non-metals are female (still doesn’t stop Carbon from being male though 
). Now, I think I will attempt taking this to the next level and see where this goes.
PS: I guess I should add that I can ‘edit’ the feelings if need be.