The contacts of an account are classified or segmented as in Groups. Each group is identified by a name that each user can define according to their criteria or utility.
Contacts in a group can be imported/exported or created in bulk or managed individually. It is important that an account maintains good group management to facilitate sendings or campaigns.
It is recommended to update the groups or contacts to increase the effectiveness of the sendings or campaigns.
The different sending methods allow to send personalized messages in bulk to all members of one or more groups. And also choose some members of a specific group.
More information at How to update or clean a database?.