Thanks all for your comments.
@dwanthny: absolutly right, but I did and still did not grasp the intention of the metadata plugboard. The answer from DiapDealer sparked the "aha" moment. Let me explain.
I already used for ages the following template when "saving books to disk" (via Preferences > Saving books to disk):
{author_sort[0]}/{author_sort}/{author_sort} - [{series} {series_index:0>2s}] ({pubdate}) {title}
Something similar, I use for "sending books to devices":
{author_sort}/({pubdate}) {title} - {series} {series_index:0>2s}
For the moment, both templates gives me the result I need.
Now enter the metadata plugboard: I never understand what that could add when the above templates already did the trick. Now I understand that it offers an
additional way of manipulating the resulting file names.
Remains the order question: will the metadata plugboard templates be handled first and then the templates from "saving books to disk"/"sending books to devices" or is it the other way arround?
Further, my main question was not so much for the saving and sending resulting file name (which I already figured out), but for manipulating the tag "title"
inside the e-book format (EPUB, MOBI, etc.), for instance during a "convert book" action. The reason for this is that my prs-t1 does not use the actual file name, but uses the tag "title" from the e-book file contents.
(Yes, I know I can sort on file name too, but then I get on two rows the same title and the auther name is missing, which I could compensate for via adding author in the file name via a template, but then the file name becomes too long for the screen, etc.)
And I do get the impression, that is not (yet) possible, right?