It would serve as the master identifier for a specific reference and would allow for easy BibTeX export, while also allowing much easier import from the many EndNoters who would happily come across if they could do so easily. It is not the same as the Call Number field, which I use to track location info for the actual object to which the record refers (e.g.: L=electronic on computer, F=hardcopy in filing cabinet, S=on shelf at work office, H=on shelf in home office, etc). I think the addition of a distinct 'label' field would aid the functionality of Zotero (which I am currently inspecting for utility as a change-over from EndNote). I don't place the file into the EndNote directory tree, but rather in a separate folder (historically, because I also ran ProCite during a transition phase and kept it separate and it made it possible to have two different programs look at the same set of files - they were not 'lost' inside the directory trees of either ProC or EndN). I use the label field from EndNote as the master key to track all of my 3000+ references - it is also used as the (base) filename for the file(s) associated with the record. Currently, BibTeX export in Zotero uses Authorsurname_Year as its default BT label, but this is not sufficiently unique across entire libraries (try it). I have used 3 types of systems so far - ProCite, EndNote and a few BibTeX-based systems. This is important for consistency of reference labelling across systems. The EndNote label is used to generate the BibTeX label used for citations in BibTeX when exporting from EN to BibTeX.

Export from zotero to endnote