When you visit a file that is under version control, Emacs indicates this on the mode line. For example, ‘Bzr-1223’ says that Bazaar is used for that file, and the current revision ID is 1223.
The character between the back-end name and the revision ID indicates the status of the work file. In a merge-based version control system, a ‘-’ character indicates that the work file is unmodified, and ‘:’ indicates that it has been modified. In a lock-based system, ‘-’ indicates an unlocked file, and ‘:’ a locked file; if the file is locked by another user (for instance, ‘jim’), that is displayed as ‘RCS:jim:1.3’.
On a graphical display, you can move the mouse over this mode line indicator to pop up a “tool-tip”, which displays a more verbose description of the version control status. Pressing Mouse-1 over the indicator pops up a menu of VC commands, identical to ‘Tools / Version Control’ on the menu bar.
When Auto Revert mode (see Reverting) reverts a buffer that is
under version control, it updates the version control information in
the mode line. However, Auto Revert mode may not properly update this
information if the version control status changes without changes to
the work file, from outside the current Emacs session. If you set
t, Auto Revert mode updates
the version control status information every
auto-revert-interval seconds, even if the work file itself is
unchanged. The resulting CPU usage depends on the version control
system, but is usually not excessive.