I'm weighing in a bit late on this; I just re-read the thread. The project is to convert DC metadata to MODS, and the specific case in question is a set of letters that are declared to be related, with no specific relationship cited. So it is really irrelevant whether or not it is appropriate to declare these to be related in MODS, they have already been declared to be related via DC and that information should be preserved when converting to MODS.
My suggestion is to omit the type attribute. It isn't required. Omitting the type attribute is the way to say that items are related but the relationship is unspecified.
> -----Original Message-----
> From: Metadata Object Description Schema List <[log in to unmask]>
> On Behalf Of Talj Harper
> Sent: Tuesday, January 08, 2019 11:25 AM
> To: [log in to unmask]
> Subject: [MODS] RelatedItem for Content Relation
> Dear MODS Community --
> I am helping on a project to migrate some old Dublin Core metadata into
> MODS, and I have found myself stumped over the <relatedItem> field. I
> have a series of letters that were listed in each others relatedItem field and
> I'm trying to figure out 1) what the relationship type is and 2) whether these
> really should be listed under relatedItem at all.
> From what I can gather, the letters were originally listed as "related" because
> they share the same author, were written on the same day, and are about
> the same topic -- but sent to two separate people. The content of the letters,
> however, is not identical.
> Looking at the type attributes for <relatedItem>, there really isn't anything
> that fits. I came across an older thread in the Listerv archive about how the
> <relatedItem> field was meant for bibliographic relationships rather than
> content relations, which may be my problem here.
> Is there a way to keep this relation between the two letters in MODS? And,
> since I'm sure I will find more of these relations as I go through legacy
> metadata -- what, in general, is the best way to indicate a content relation
> between items?