Hi, this is my area on the forums - not something watched by the whole team. So sorry for delay - I was on leave.
I have not heard of any definitive scheme for SKU allocations. People do what makes most sense for their business. The trick is to come up with unique IDs for all your products. Using letters that make sense in the context of your business and staff can help people remember what a SKU is. Keeping SKUs short often makes them easier to deal with (but becomes harder as your catalog size increases).
My recommendation is to look at your range of inventory and come up with some form of ID scheme based on that. (But I don't claim to be an expert on SKU allocation strategies.)
To get more experienced opinions on SKU allocations, Google is your friend! ;-) I quite the UnderstandingE web site https://understandinge.com/lesson/ss-0001/