What is the Territorial Army in the novel, Under Fire?

The French Volunteer Army consists of trained reservists who can be called up in an emergency, the Territorials are mocked by Bertrand's Squad when they march in to fortify the second line and help maintain the communication trenches. Most are "stunted and elderly," dusty, "broken-winded," wearing patched-up greatcoats. Tirette and Barque, the squad's "twin wags," make fun of them.