When is a part not a part?

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Excellent! Thanks @Griomo. I knew about Op130 but had forgotten. The others add to my education .

In terms of how they play into the current discussion (regarding works with multiple parent works):

• Große Fuge: This is shown as a stand-alone work, separated from its original parent, which contains the late allegro as well as all the other parts. I think this is correct since it was subsequently published as a separate work (Op.133)
• As regards the Mahler, all the parts of the different versions are separate records in MB. I am assuming that there are, at least, subtle orchestration differences throughout so that none of the parts are the same as the corresponding parts of previous versions.
• Milhaud: I’m not familiar with this work, but as far as the MB database structure is concerned, none of the parts are shown as “part of” more than one parent work. The linked thread took me to Work movement reused in another work - Relationship type?. which I realise is almost identical to the question I am now trying to resolve. Unfortunately, it did not get answered. In that post, @alex_s7 asks what approach to use where a composer re-uses one of (their own) work movements:

@alex_s7

• Create only one work for the reused movement, link it as a part to both “container” works: in such case the name of the movement is not in line with name of one of “container” works (different catalogue number).
• Create two separate works for reused movement, add a relationship between them which indicates that the work has been reused. Which relationship may be used in such case?
• As a last resort, create two separate works for reused movement and add an annotation with descriptive text.

• I think there is a fourth option: Place only the new/revised works in the later “container” work. That said, I think (in an ideal world) option 2 would be best, but it would require an additional “(re)used in” attribute. Failing that, I would go for option 3 (with disambiguation to make it clear that the movement is just re-used).
• Atterburg: Symphony 2 (another new one to me!). Here the original work in MB has no parts - the parts are just movements of the later revision.
• The Ben Johnston work would be an excellent example (when the movements of the sonata are played in an alternate order, the piece is titled Grindlemusic) . Unfortunately, Grindlemusic is not in MB as a separate work.

So, to sum up, although the specific quoted works are all excellent examples of re-used movements, none of them have been implemented in MB as a part belonging to more than one work. I haven’t investigated all the suggested operas, but I looked at Fidelio and, again, that is implemented in MB by using different work records for the parts of the revisions - i.e. no parts with more than one parent.