I’m not a linguist, but to me, langauge is fluid, in that it’s fine to use it any way you want, so long as the people involved can interpret it as intended.

Which means when you write a note in shorthand for yourself, so long as you can come back to it and decipher it at whatever point you need it then it’s fine.

If someone were to happen to come across it then there is a concern that they may interpret it wrong. As a silly example: If “I will fuck your mom” was your code for “I will pick up milk and eggs from the grocery store”, you can see how people (which could include you later) can interpret your message incorrectly, and you should pick a better shorthand in that case.

When communicating with others you’ll have contexts of what kind of things are relevant, like PoS may mean different things if you are talking to a store manager vs. your buddy, you have to be sure your listener or reader gets the correct version.