ISBN numbers & print proofs
Originally, I thought the whole story was, "Don't use purchased ISBN numbers when uploading test copies for you to review print proofs."
Now I recognize that 1) this was a Blurb-specific admonition, and 2) I need to slow down and read the fine print even when I'm feeling overwhelmed.
As part of the journey to getting a print version of any book out, the wise author/publisher will order physical proofs to review. This is where you determine if the style of the cover printing (e.g., matte vs glossy), the weight of the paper, the quality of the printing press, etc., meets your needs.
When you upload your files to the company printing your books, they will need a unique SKU or ISBN number to identify your book. I screwed up this process and it was expensive and time-consuming to resolve it. So read this before uploading anything so you don't have the same problems.
If using Blurb:
On Blurb, any changes to paper weight or printing style requires you to re-upload your files and create a new entry. They don't charge a set-up fee, so the only cost is to order the book (and don't forget to factor in the 4-5 days waiting time for Print On Demand).
My screw-up here was to attach my purchased ISBN to the first one I uploaded, before I had confirmed the paper and printing style etc that I wanted to use. It was a pain to get that ISBN released from the bad version so that I could use it for the real one.
Blurb will provide a free SKU which you can use for non-distributed books (books which aren't going to be put on sale anywhere). This is what you want to use.
It'll just be a headache later as you try to explain that you never put that book up for sale so please just transfer that ISBN to the final version which you do you want to put up for sale.
I am learning this the hard way so you don't have to.
If using IngramSpark:
Ingram has a different method (and the difference is what bit me). They charge a set-up fee for adding a new book to your account, but will allow you to make changes to the paper weight, etc., after you've created the file until you're happy with it and ready to set it up for sale.
For Ingram, you want to use the ISBN of the final book when you first upload it. If you don't, then you'll just have to pay for another set-up fee to upload the book with the correct number. So... Caution to all, especially those who are handling the details of self-publishing print versions in weird hours when you may be sleepy and more likely to overlook important details.