"reserved for future use" does not mean anything for a format that is tight to a version. In this format they are truly irrelevant. Should the format change, well, the format will change, and won't preserve compatibility, so "reserved for future use" is irrelevant ^^ I thought this one trough, I had also thought about the "future use".

> btw. I suppose the necessity to make the col diffs signed made the 1-byte line and col diff impossible.

Yes, I produced stats of what diffs are most present, and diffs of column in the range [-64;63] are not uncommon. Apparently more common than +1 in line diff. +1 in line is also not more common than -1, -2, +2 and +3. So optimizing for +1 in line was not useful. This is the format that gave me the best compression on my tests (and among the formats I tried).