Does the FED rulebook or casebook define "unnocupied and occupied bases"

[mcrowder]

HTBT, but neither is a balk.

A base that was occupied at TOP is always legit to throw to. You are also allowed to throw to a base where a play can be made.

Now ... 2 STEPS off 1st, and a throw to 2nd? Balk - no play was being made. There's no line in the sand for the runner to cross for you to say "Balk or no balk". It's your judgement - is a play being made?