A telephone extension is a device (like a telephone or softphone) that registers to the Press8 PBX using a 7-digit "extension" number and a password. 

Additionally, each configured telephone extension can be used for calling other local extensions internally by just dialing the last 3 digits of the 7 digit extension number, without the need for dialing "outside" using a public telephone number.

Your telephone extensions can be managed in your Press 8 Telecom Control Panel, by navigating to: Features=>Telephone Lines.