Federal Court: Nature of Suit
The federal courts require plaintiffs to check a box indicating Nature of Suit, such as 110 Insurance contract or 550 Prisoner civil rights petition. PACER's user manual provides a complete list of Nature of Suit codes.
NoS codes can help you search for cases by type but keep in mind:
As a result, it's usually best to experiment with both keyword and NoS searches.
State Court: Case Type and Key Nature of Suit
Most state courts do not have classification systems as detailed as Nature of Suit, but they typically provide some general description of case type (e.g. criminal misdemeanor, civil family support, unlawful detainer, criminal traffic, state bar resignation).
Additionally, Westlaw has developed an unofficial system called Key Nature of Suit in which it assigns NoS type codes to state cases. Westlaw's help page provide a complete list of Key Nature of Suit codes.