In general it's the same as in other parts of MB: one song = one work
The work-concept in classical MB differs slightly from "popular" MB, because it describes something that exists in physical form (a score). Hence we (can) separate different versions of the same work (popular arrangements & so on).
A new work should be created for every self-contained movement in the score & linked to the parent work.
Don't create new works for sections that are clearly meant to be performed as one entity by the composer.
That includes titled sections in symphonic poems, scenic instructions in operas and so on. New work entries should not be created to follow splits in audio recordings. See pseudo works for exceptions.
Sometimes it's not entirely clear in the score if it's a sub-movement part or a new movement. In rare cases different editions of the same work can split the music in different ways. Try to follow the most authoritative source, the most recent urtext score, for example.
In general, do not enter more than one set of works as sub-parts of the same main work. See different versions and arrangements for exceptions.
Generic titled collections
Sometimes, a composer had works published together in collections; "3 songs/sonatas" and similar.
Prelude & fugue has parts "Prelude" and "Fugue", because they are musical entities of their own Menuetto - Trio has no parts Gavotte I / II Symphony Song-cycle Collections
Sometimes we need to make an exception for partial works that are often recorded. A list of exception would include popular arias & choruses from operas (operas without numbers, that is). Another possible category of "convenienve works" is programmatic notes in the score that are not self-contained movements, but frequently used as track splits, ex. Also sprach Zarathustra.
UI workaround for sub-part titles
Currently we have to repeat the title of the main work in all parts. It's "only" an UI change to fix this, since works are not used for tagging. It's recordings that always need the main work inside the actual title field. We would need the parent work to be shown close to the title of the sub-part, like Mut ~ Work (part of Winterreise) or something. And the main work is also needed in search results.
Always use a colon as delimiter, regardless of language (or latin script only?)