The global tempo and time signature can be overridden by custom tempo and time signature that can be set for each rackspace:

Check the Rackspace Properties chapter for more information.
When you enter the Setlists view, the global tempo and time signature can be overridden by a custom tempo and time signature that can be set for each song:

Refer to the Song Properties chapter to learn more.
Song parts can also have their overrides and they represent the highest priority in the override order:

This option is configured in the Song Part Properties dialog.
Note: if you add the Set Global BPM action in the Song Part properties dialog, it will override the Override global tempo setting described above.