From Wikimon

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 two 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.

Field	Kanji/Kana	Romanization	Symbol	Description
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-based digimon.

Field Emblems