Correction:
> I'm assuming NMTOKEN ...
> so the subFlows value can be of type NMTOKENS. If we assume NMTOKEN, then we should have something like:
<unit id='a1' parent='a3'>
<source>Title text</source>
</unit>
<unit id='a2' parent='a3'>
<source>Alternate text</source>
</unit>
<unit id='a3'>
<source>This is an image: <ph id='1' subFlows='a1 a2'/>.</source>
</unit> As NMTOKEN must start with a letter. To be honest I don't like that restriction at all. Maybe we should define the subFlows as a comma-separated value. That would still be compatible with NMTOKEN values (which can't have comma I think) and less-restricted id values. So you could have subFlows="1,2" or subFlows="a1,a2", etc. -ys