A Group A Incident contains both offense segments with information about the offenses related to the incident and arrest segments which contain information about the arrest offense and other arrest information associated with the arrest. An offense segment can only contain Group A offenses but a Group A incident can have an arrest segment with Group A or Group B arrest offenses.
A Group B arrest only includes arrest information and can only contain Group B arrest offenses.
When a person is arrested, they may be charged with multiple offenses. However, when submitting this arrest to NIBRS, only one arrest segment (with one arrest offense) per person can be submitted. This rule applies equally to Group A and Group B arrests.
Within a Group A incident, all offenses that were committed are reported in the Offense segment. These are all considered to be cleared as soon as any arrest is submitted as part of this incident, so there is no need to track all the charges with the arrestee.
When a person is arrested on multiple Group B offenses, again only a single arrest segment should be submitted.
Typically an RMS will need to capture all the offenses which an arrestee is charged with. This is important for courts, criminal history, etc. Depending on the RMS, it may leave the selection of which offense to include in NIBRS to the person submitting the data, or it may have some kind of built-in hierarchy rule.
There is one exception to the rule above. In some cases, a single arrest may clear multiple Group A incidents (e.g. a string of burglaries). In this case, a single arrest segment should be attached to each Group A incident being cleared by the arrest. In order to avoid counting the arrestee multiple times, it is necessary to populate the "Multiple Arrest Indicator" field with the value "M" for all records except one, which will have the value "C" instead.