thank you for your reply!
You're right: I was wrong because I've always seen I'm able to chmod +t a file but I never tested that it has no effect

... never tried it before.
Thanks for clarifying this for me
Anyhow I'm still trying to understand how to inherit the sticky bit on new folders (he he he

) under a folder with sticky bit on.
I explain it better:
I chmod 1770 a test_folder
If I set the general umask, or acl mask specifically on it, to be 770
users will create new folders with 770 permissions of course but no sticky bit!
umask and mask seem to work only with the last three digits .. or am I wrong? ... so no way to inherit the sticky bit even if I set a mask with 4 digits as for example 6007
So how to permit users to create new folder already with the sticky bit activated under test_folder???
Maybe only a cron job can chmod 1770 -R test_folder.
But has this got a sense?