A more thorough explanation:

Definition: PCA stands for Posse Comitatus Act. It is a federal law that limits the use of military personnel in domestic law enforcement activities.

For example, if there is a riot or civil disturbance, the military cannot be used to enforce the law unless authorized by Congress or the Constitution.

This law was passed in 1878 after the Civil War to prevent the federal government from using military force to enforce laws against citizens. The PCA helps to maintain the separation between the military and civilian law enforcement.