Group

"Group" may refer to a:

"Bundle" - items are released together, acquiring the bundle or package gifts the player all items in the package at once. Such bundles are usually announced by game staff. (official)

"Collection:" Items are clearly related but methods of acquisition might be a bit different and don't qualify for other types of groups. Generally applies to Prize Box items. (semi-official)

Family: A user-based grouping of entities that share similar themes, like "food." (unofficial)

Set: items are very obviously related because they share similar names, stats, and abilities. (semi-official)

Items can be related in other ways, but these are generally less distinct and are usually included in a category of some kind, such as "Weapons that can inflict Shock," and so on.