A Digimon's Field (フィールド Fīrudo) is an overall branching group of Digimon that each one belongs to. There are ten different fields, each with a different theme. Digimon can have more than one field or none. Field represents three things.
First of all, Digimon of the same field are likely to be able to evolve into similar forms, making it slightly easier to construct and group cards together at a glance. In the Card Game, fields are symbolized by a small tile with a two or three letter abreviation, with one large letter in the upper left corner and the other large letter in the lower right corner, sometimes with a third smaller letter imediately next to it.
Secondly, in some media like V-Tamer, a field can represent the habitat that the Digimon occupies. A Digimon in its own field can get an advantage in the card game.
The Digimon Pendulums are also based on those fields ( they are indeed the source of them and were later adapted into the cards ), with them introducing the first themed Evolution trees, as opposed to the rather random Evolution trees of the original V-Pets.
|Nature Spirits||ネイチャースピリッツ||Neichā Supirittsu||NSp||Generic monsters and beasts. Also represents grasslands and canyons.|
|Deep Savers||ディープセイバーズ||Dīpu Seibāzu||DS||Aquatic or ice-related Digimon. Can represent the ocean or arctic.|
|Nightmare Soldiers||ナイトメアソルジャーズ||Naitomea Sorujāzu||NSo||Darkness or spirit based Digimon, or Digimon based on traditional "Halloween" monsters like vampires, werewolves, or Japanese youkai and demons. Can represent haunted or cursed areas.|
|Wind Guardians||ウィンドガーディアンズ||Uindo Gādianzu||WG||Flying, air, or plant based Digimon. Can represent forests and the sky.|
|Metal Empire||メタルエンパイア||Metaru Enpaia||ME||Machine, cyborg, or other mechanical Digimon. Can represent cities and factories.|
|Unknown||アンノウン||Announ||UK||Mutant, unusual, or strange Digimon. Can represent junkyards and other strange places.|
|Dark Area||ダークエリア||Dāku Eria||DA||Villanous, demonic, terrifying, evil Digimon. Can represent the Dark Area of the Digital World.|
|Virus Busters||ウィルスバスターズ||Wirusu Basutāzu||VB||Heroic or angelic Digimon, often includes main characters from the anime or manga. Can represent the sky or heavens.|
|Dragon's Roar||ドラゴンズロア||Doragonzu Roa||DR||Draconic Digimon.|
|Jungle Troopers||ジャングル・トルーパーズ||Janguru Torūpāzu||JT||Plant or Insect based Digimon.|