Digimon are sorted into "Fields" based on their physiology, habitat or powers. Many Digimon have been associated with more than one Field accross time and different media.
Fields were first introduced to the franchise through the orignal Pendulum v-pet series starting 1998 and the original Card Game (AKA Hyper Colosseum) in starting 1999.
The Pendulums versions 1-5 were Nature Spirits, Deep Savers, Nightmare Soldiers, and Metal Empire, with the addition of Pendulum ZERO, Virus Busters.
In Hyper Colosseum, up to Booster 6, the field was initially only indicated by the background, but starting Booster 7 a symbol was added. Later releases eventually opted to use plain text.
This line of Fields are Nature Spirits [NSp], Deep Savers [DS], Nightmare Soldiers [NSo], Wind Guardians [WG], Metal Empire [ME], Virus Busters [VB], Unknown [UK] and Dark Area [DA].
In 2006, the new Digimon Card Game alpha released, and dropped the Unknown and Dark Area Fields, and added Dragon's Roar [DR] and Jungle Troopers [JT].
—Nature Spirits [NSp]
This field covers many Digimon living in greenery, around lakes or desert areas. They're often considered simple and gentle.
Starting the 2006-Savers-era this mostly only contains beast-like Digimon.
—Deep Savers [DS]
These Digimon live in the Net Ocean of the World. With the 06-era all water based Digimon started being included in this.
—Nightmare Soldiers [NSo]
Ghost and Demon type Digimon are in this field, as well as Digimon releated to Halloween. Many of them have a relation to darkness, but some are related to super natural themes.
—Wind Guardians [WG]
This field contains Bird Digimon, and generally Digimon associated with air or sky. Sometimes Vegetation-type Digimon are also included.
—Metal Empire [ME]
Machine, Cyborg, or Mutant type Digimon belong to this field. Later many "material" based Digimon or Filth Digimon get sorted into this field too.
—Virus Busters [VB]
Angelic and Holy Beast Digimon are in this Field.
—Unknown [UK]
This Field would include Mutants or Filth type Digimon. While a lot of modern media spreads these across other Fields, the Digimon Web still acknowledges it.
—Dark Area [DA]
Evil Digimon, namely Villains from the series, are in this field.
—Dragon's Roar [DR]
As mentioned above, this Field got introduced in 2006 and includes Dragon and Dinosaur type Digimon previously included in various other Fields.
—Jungle Troopers [JT]
Just like Dragon's Roar, Jungle Troopers had several Digimon from Nature Spirits and Wind Guardians added to it. It covers Insectoid and Vegetation Digimon.
While most Games don't include Fields, but rather some form of elemental classes based on primary attacks, Digimon World Data Squad does include the Fields as such, and the the DS games have "Species" categories that line up with the Fields: NSp > Beast, DS > Aquan, NSo > Dark, WG > Bird, ME > Machine, VB > Holy, DR > Dragon, JT > Plant/Insect. While the early DS games had one catergory for Insects and Plants, starting with Lost Evolution a Plant species was added.
What kind of elemental types are included and how they relate to each other varies strongly from game to game, the only constant being Fire. There's also multiple ones that overlap. Water and Ice are often grouped up, Electricity often includes Wind or vice versa. Nature, Earth and Combat often cover the same or similar ground too.
These elements also don't always refer to the Digimon themselves, but often only attack moves. And in Digimon World 3 and 4, and Survive, the Digimon don't have elemental types associated with them, but instead their resistance to each one.
