A crossclaim is a claim asserted between codefendants or coplaintiffs in a case and that relates to the subject of the original claim or counterclaim according to Black's Law Dictionary. A cross claim is filed against someone who is a co-defendant or co-plaintiff to the party who originates the crossclaim. In common law, a crossclaim is a demand made in a pleading that is filed against a party which is on the "same side" of the lawsuit.
In the Federal Rules of Civil Procedure this is codified in Rule 13(g). In the federal rules, a crossclaim is proper if it relates to a matter of the original jurisdiction. Proper jurisdiction is determined by a finding of whether the suit that is being initiated arises from the same transaction or occurrence that is the subject matter of the suit.
The policy for allowing crossclaims is that they promote efficiency and consistency. Furthermore, the same underlying facts will be litigated on the main claim as well as on the crossclaim preserving efficiency in the judicial system by resolving multiple claims that might arise between the parties as opposed to courts trying each claim individually and re-litigating the same facts. Furthermore, this will prevent inconsistent verdicts that might harm the public perception of the judicial process. Finally, because crossclaims are not mandatory, they allow the moving party the opportunity to sue later. The plaintiff is the master of his complaint, and this also holds true in crossclaims.