User blog comment:SuperSpruce/The T function part 3: Tetrational Arrays/@comment-35470197-20190409011116/@comment-36984051-20190409101234

A dimensional separator is something inside the curly braces that cannot be converted into a comma. a doesn’t just have to be a natural number, a can be an entire array!

For your second question: Yes, the next entry after the 0 of [0,[1],2] is 1, and since a [ is immediately before the 1, go to step Bb.