I understood how book bands worked in KS1 - and DS as regularly moved up when he displayed sufficient decoding ability and comprehension.
He's now in Y3 in a different school and the books he is on (lime band - different scheme to old school) are much simpler in terms of decoding ability required but harder in terms of comprehension than what he was reading at the end of Y2. DS can decode them very easily - but IMO his comprehension is actually good too - he reads with expression, he laughs in the right places, he often reads ahead and gives me a summary of what is coming next ...
He tells me the next band is dark red (?), but I don't understand at what point he moves up? Clearly he's at a level where it's not just about being able to work out what the words say, but what else is being looked for?
(I do realise the logical answer is to talk to his teacher, but I only see her irregularly and she's usually surrouned by other parents so it's hard to do so. Also, although she does guided reading with him, she's never read individually with him - is this normal for Y3 - he has read individually with other teachers and TAs?)