In play-by-mail (PBEM), players play over the Internet, but do so in an alternating and asynchronous fashion. When one user makes a move (i.e., completes a turn), information about their moves is sent to a competitor via e-mail, after which the competitor - at any time - begins their own turn. This form of multiplayer does not require players to sit in front of a computer (or console) at the same time, and is primarily found in turn-based strategy games (usually niche war games).
Field Commander is a turn-based war strategy taking place in the near future. The creators of the game were clearly modelled in their work on Advance Wars, a great hits available on a competitive device.