Application field in Document Properties

Does anyone know what the Application field in the Description tab of the PDF Document Properties is for? I am asking because my office uses a custom merge process to generate packets of multiple PDFs and I encountered an issue the other day where the packet generation failed for 6 PDFs and the only thing they had in common was that none of them had anything listed in the Application field. I did some additional looking at various other PDFs I have on my PC and all I looked at had something showing in the Application field. It appears that having no value in this field causes my packets to fail but I don't know what this field is intended to represent or how to populate it if it is blank. Anyone know more about this field?