A more thorough explanation:

Definition: REG. PL. is an abbreviation for "regula placitandi," which is a historical Latin term that refers to a rule of pleading in court. It was used in the past to regulate the way legal cases were presented and argued.

Example: In medieval England, the regula placitandi was an important part of the legal system. It set out the specific procedures and language that lawyers had to use when presenting their cases in court. For example, they had to follow a strict format for stating their arguments and had to use certain Latin phrases to make their points.

This example illustrates how the regula placitandi was a specific rule that governed the way legal cases were conducted in the past. It shows how important it was for lawyers to follow this rule in order to be successful in court.