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.
The window two readings of one feature must fall in to be collated: how far apart they may lie at either end, and where that window is centred.
The centre is worth stating. Two copies of one roll differ by a systematic offset as well as by scatter, and a window centred on nothing has to be widened by the whole of that offset before it admits what the offset alone displaces. Naming the offset lets the tolerance stand for the scatter only. A window that names none is centred on nothing, which is what every window written before the offset was held here means.