> Thanks for the clarification. Then the new element should
> live in a module, as it is not essential for the
> translation cycle.
Technically I don't think it can live in a module: It requires to be nested into the structure of <file> and to live among the list of <unit>, not just set at the end.
It's also an important aspect of parsing the document: user agents supporting only the core should be ready to deal with units at various levels, otherwise they may not handle them properly if a non-core element is mixed with them.
And just like preserving the number and order of the units is important for merging back, preserving its structure can be as well.
Cheers,
-yves
