It's been a few years since I "enjoyed" writing code to mangle Microsoft Office documents, but I've been getting back into such things at work lately. My team and I had lots of fun yesterday tracking down the source of a bug in POI which led to Excel mangling comments when the file was saved. Frankly, I wasn't sure to be delighted or disgusted that the "fix" was changing an ID value in the XML output. Yes, it would seem that a so-called arbitrary identifier has significance to Excel beyond cross-referencing elements in the same file, and using the same not-so-arbitrary identifier written by Excel itself dissuades it from trashing the file on save.