There aren't a lot of rules for Tag Acronyms, but the goals behind the Tag Acronym are:
They're short (less than 20 characters, ideally less than 10)
They make sense to you
It's important that the Tag Acronyms make sense to you, especially if you use them for filtering the Results by Group. It's not ideal if you can't remember if "B" stands for "Biology" or "Biochemistry."
You can use letters, numbers, and punctuation (the most common punctuation are - and .).
Note: Tags used to be called Groups.