How many readings the window was measured on; none where it fell back on the default.
OptionaloffsetThe same at the end of a feature, in the same direction.
OptionaloffsetHow much later the derived version puts the start of a feature than the version it is read against, where the two differ systematically. Nothing where they do not.
The direction is the one a collation measures in, the child less
the parent, and it is not symmetric: stored the other way round
the window sits on the wrong side of the readings and merges what
it should separate. A measurement taken over the parent's copies
gives the negative of this, which is what toleranceAcross asks
the side for.
How far they may lie apart at the end, measured from offsetEnd.
How far the two readings may lie apart at the start of a feature, measured from offsetStart.
How far two copies may put one reading apart, and where that window is centred: the systematic difference between the two and the scatter about it, as
scatter.tsmeasures a derivation's.