I've elected to store the "transcriptional" boolean in the form of a <desc> tag like this, inside the <rendition> element:

<desc>transcriptional</desc>

I have this correctly saving and reloading from the file; what isn't done yet is the distinction between the use of @facs and @corresp for linking. I'll have to do this by giving the annotation list a method pointer which can be set to point to TAnnCatList.GetTranscriptionalFromCatID method. I'll have to make sure that binding is set up by the TIMTDoc, in its Create method. Then the annotation list can find that information and decide which attribute it needs to use when saving the file.