auto-revert-tail-mode-text is a variable defined in `autorevert.el'.
Its value is
" Tail"

String to display in the mode line when Auto-Revert Tail Mode is active.

(When the string is not empty, make sure that it has a leading space.)

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.