This is a major change to the way block metadata is handled within the PM core. This separates variant metadata (which really ought to be part of the ID) from state metadata, and in a couple of cases flattens separate states of blocks together.
The result of this is that invalid variants can be much more easily detected, and additionally state handling is much cleaner since meta is only needed at the serialize layer instead of throughout the code.
long overdue... this isn't quite as extensible as the original api3/blocks system was, but this is primarily intended to replace Item->useOn(). If plugins want to use it it can be extended later on.
