A more thorough explanation:

Term: Primer
Definition: Primer means something that is first or primary. For example, when someone gets the first choice of something, it can be called a primer choice. In law, primer seisin means the first possession of land. Another example is the primer election, which is the first choice of land by the eldest coparcener when an estate is divided. The term primer is often used in legal contexts to refer to something that is the first or primary choice.