A "peripheral_vessels.xml" file was created to house vessels mentioned in files other than the despatches. For example, in Captain Cook's biography, we might mention his ship, Discovery, which does not appear in the despatches, at least not in the content transcribed currently.
As we discussed as a team, it seems odd that the online reader should encounter some vessels tagged and others not. After all, readers do not know which vessels occur in the letters and which do not. The peripheral-vessels file solves cures this potential for confusion.
Lastly, should a vessel that appears in the peripheral-vessels file one day be discovered elsewhere in the future, say, if the enclosures are eventually transcribed, then we would move the respective vessel entry over to the "vessels.xml" file, a simple copy/paste operation.