Is it possible to tag a section of an RTF document?

I have some RTF documents in my database, transcribed documents, which are quite long. I’d like to tag certain sections or quotes with different subject tags. Is it possible to tag a portion of a document, as opposed to the whole thing?

No, tags belong to the whole document. (Tags in fact are groups, and tagging in DEVONthink makes a replicant of that document in that tag group). You could, of course, split the document and tag the split-off components individually.