Both seem to be used in the database, but the correct version appears to be “30 kleinkunst klassiekers, deel 5” with the lowercase d.
The justification for this can be found in the series numbering section of the title style guidelines. Here we find a similar case following French language guidelines:
ETA: Could you change the title of your OP to something more descriptive of your issue? Then generations of future editors can benefit from the pearls of wisdom exchanged here
The title of the release is 30 kleinkunst klassiekers.
Deel is not a part of the title it’s what we call a Bijzin
Because MB made the rule that there should be a comma after the tiltle we now have the problem that it became 1 sentence, therefor with a d and not a D. But they are two different sentences so I think it should be Deel and not deel
Anyway, MB considers series numbering part of the title:
When a word such as “volume” (“vol.”) or “part” (“pt.”) is used in a title to indicate its position within a series (of releases, tracks, etc.), insert a comma before that word.
Hence, under MB definitions, “deel” and “volume” get to be lowercased in non-capitalizing languages. And MB makes the rules here
ETA: I took the liberty of modifying the title slightly further and putting it in the appropriate category. Hope you are OK with that.
Thank you for explaining, I will change it when I see it
ETA: I took the liberty of modifying the title slightly further and putting it in the appropriate category. Hope you are OK with that. - Sure I’m oke with that, you’re English is better then mine
The release itself contains a language error:
Correct Dutch would be: 30 kleinkunstklassiekers (or alternatively: 30 kleinkunst-klassiekers)
It’s the infamous space error (spatiefout) which is in Dutch completely different than English. (“Spatie fout” translated in English would become: “Space, Error”)