The same data set can be used for multiple different mailmerges. So, you could have a separate mailmerge main document for each case type. And you could have separate mailmerge main documents for each of the documents a given case type might require.

With letter merges, however, it's easy enough for a given mailmerge main document to incorporate all the document types you need for the corresponding case.

Where different case types use the same stationery, you might even have just the one copy of that stationery (easier to maintain), together the any required mergefields, and use INCLUDETEXT fields to import it into the others.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]