with more experience with the interface, I think I have an answer for my 2nd question from yesterday as well:
2) If I should have a total of 25 domains available, and 7 reported to be used, what does "You have left only 0 domain(s)" mean?
I think ISPConfig is subtracting from the total domains available for assignment for the reseller the number of domains the hostingplan is assigned. While this figure is not shown in the main reseller login view, this actually makes sense to me now (I just have to keep a few numbers in my head) ;-)
1) admin creates a reseller, allows reseller 50 domains, total.
2) reseller creates a hosting plan that provides the client a maximum of 40 domains.
3) reseller cannot create another client with more than 10 domains allocated, as this will take them over the 50 domains admin user allocated reseller in the first place - even though the domains are not physically in use yet.