After downloading a few files from one single torrent,
I switched some files in priority to OFF.
I was surprised to see, there is still a (r) for reading
coming up at this files.
So that means they are not truly OFF.
I moved those OFF files to an external HD,
because running out of storage space on HD.
After about 2 hours the computer crashed
and I don't know the reason.
Could be related ? I don't know.
Maybe next time I should restart Tixati
after switching OFF and moving files.

I think this is by design, Tixati seeds all available pieces. This is sometimes useful for seeding incomplete files. Most likely other clients do that too, but it's harder to tell because they don't show as many information as Tixati.

If your partial seeding transfer has files which fit within the cross-file pieces you are downloading for the files you DO WANT... you will still see them
They may even show 100% and "complete" even though you don't want them.
Don't remove/delete them unless you want to seed bad data to others.