The commission was the proof the privateer was not a pirate. It usually limited activity to one particular ship, and specified officers, for a specified period of time. Typically, the owners or captain would be required to post a performance bond. The commission also dictated the expected nationality of potential prize ships under the terms of the war. At sea, the privateer captain was obliged to produce the commission to a potential prize ship's captain as evidence of the legitimacy of their prize claim. If the nationality of a prize was not the enemy of the commissioning sovereign, the privateer could not claim the ship as a prize. Doing so would be an act of piracy.
In British law, under the Offences at Sea Act 1536, piracy was an act of treason, or raiding a ship without a valid commission. By the late 17th century, the prosecution of privateers loyal to the usurped King James II for piracy began to shift the legal framework of piracy away from treason towards crime against property. As a result, privateering commissions became a matter of national discretion.

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