What is the Default tab for?

Every Zone Set has a default tab. Dialer uses this default zone entry if no time zone entries in the Zone Set match the zone of the contact.  A Default tab is added to any newly created Zone Set. You can modify time slots in the Default zone like any other zone entry. The difference is that the Default Zone cannot be copied, deleted or renamed.

By default, this tab defines hours that are acceptable for all zones in North America (Atlantic to PST – ignoring Hawaii). That is GMT-4 to GMT-8 which equates to Noon to 5:00 p.m. based on a 8:00 a.m.—9:00 p.m. allowable calling hours.