You tell them what film you use, they send you six unexposed sheets of that film. You process that film in your chemistry for six different specified times and send it to back them. So if you use replenished developer to process your film their tests and the film speed/development time information they send you will be based on your film in that developer.
If you click on "BTZS Products" on the left-hand side of their home page and then scroll down you'll see a reference to their testing service. It doesn't go into any detail that I saw so if you have questions about it I guess you'd have to contact them. But I'm pretty sure they still perform this service, I believe I saw a reference to it in a post here a few weeks ago from Fred Newman, the owner of The View Camera Store.
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