<expan> should contain the whole of a word that has been expanded. 
<expan>Mister</expan> when the source says "Mr."

<ex> should contain the bits of an expansion which are not present in 
the source text but have been added to it by an editor.

The truly obsessive will also wish to mark (using <am>) the full stop 
which signals that "Mr." is an abbreviation of course.


Of course, it might be that (this is for Bertrand) you think "Mr." is 
actually an abbreviation for "Monseigneur", in which case you'd have


Or, if unable to decide, a <choice> containing both, and maybe also an 
<abbr> holding the original form just for fun. Though in that case, 
where to put the <am> becomes a bit trickier.

On 19/07/17 17:42, MLH wrote:
> Could anyone explain the rationale for combining <ex> and <expan> in 
> this way? (for someone not as familiar with the transcription module 
> as he probably should be!)
> Thanks,
> Matthew