Keep in mind that this is the website from Deutsche Bahn, which operates a lot of trains, regional trains and local trains. But not every public transport is from Deutsche Bahn. The 9-Euro-Ticket is valid for all public local transport, too - not just for the Deutsche Bahn.
The ticket is for local transport operators, too. One can also buy the ticket there. A subscription ticket will be automatically converted. One can for example buy it at the local Verkehrsverbund (transit district), which here serves in Hamburg several million people with all kinds of regional trains, city trains, underground trains, busses, ferries, ...
Thus one can use all (!!!) local and regional public transport for 9 Euro for a month, 24 hours a day, for all of Germany, for all public transport operators. The is done for three months, each month costs 9 Euro.