I'm sure this question will be answered when your PDF Seminar on "Publishing" is on the market, but in the meantime, I'm interested in the requirements for, and the differences between, ISBN and ISSN numbers and why they exist. I know that the ISBN is used on books, and that ISSN is used on magazines. I noticed that LensWork magazines have an ISSN and so do the LensWork Extended PDFs [but different numbers for the same issue] ...but the folios don't have any such numbering shown. Could you explain why?
Basically, these numbers are unique identifiers that allow resellers (i.e., stores) to scan the barcode and identify the unique item. You can learn all you need to know from RR Bowker, the company that issues the numbers. That's really about it, except for one other, very important distinction.
ISBN numbers are used for books; ISSN numbers are used for periodicals. Why distinguish? As Mark Twain used to drawl, "Da gov'mint." The US Postal Service has a special class of mail delivery for "periodicals" — a fancy term for magazines. Why? I don't know, but I suspect it's a classic case of special interest lobbying. The magazine business has a cheaper rate for mailing stuff than regular mail. It saves publishers (and subscribers) a ton of money.
LensWork — our bimonthly, subscription-based publication — qualifies according to USPS rules as a periodical and can benefit from the cheaper postal rates. (This is domestic mail only, of course.) Part of the qualification is that it is identified with an ISSN number, again which it qualifies for. We all save money.
LensWork Extended — our bimonthly, subscription-based publication — however, does not qualify as a periodical because the US Postal Service is stupid —uh, er, I mean, behind the curve. You see, it's not made of paper, is not chock full of ads, and doesn't feel like all the other "periodicals." Being a computer disc means that it can't be officially designated a "periodical." So, they have time and time again rejected our request to have it qualified as a periodical. We keep pushing and they keep resisting. This means it must mail to subscribers at a higher postal rate. Thankfully, it's lightweight or the postage would go through the roof.
This is also why LensWork Extended has an ISBN number and barcode rather than the ISSN number you see on LensWork.
This is also why LensWork and LensWork Extended do not mail together in the same envelope.This is also why we have certain feelings for the USPS that I'd best not include in this post.
We love the post office, we love the post office, we love the post office, we love the post office, we love the post office, we love the post office, we love the post office, we love the post office, we love the post office, we love the post office, we love the post office, we love the post office. Honest, we really, really do.