There are 5 blocks in each SWIFT-MT message, so why in the specification, it needs up to 3 alphanumeric letters for the block indentifier?

Question

I read from this post

and in the Standards MT November 2022 - General Information, page #48

So there are max at 5 blocks for each message. So can anybody please tell why the specification needs up to 3 alphanumeric letters for the block indentifier?

ex: description of the SWIFT MT Message Block 1 (same for other blocks), the block identifier content is '3c'.