A more thorough explanation:

A parson is a spiritual leader and the head of a parish. They are also known as a rector. A rector is a presiding officer of a parish. A lay rector is a non-clerical rector, and a rector sinecure is a rector who does not have the cure of souls.

For example, a parson may lead a church service and provide spiritual guidance to members of their parish. They may also oversee the administration of the church and its activities.

Rectum means "right" in Latin. It can also refer to a trial or accusation.

For example, in a court case, the defendant may face a rectum or accusation of wrongdoing. The rectum in this case refers to the legal process of determining guilt or innocence.

Rectus means "straight" or "direct" in Latin. It is often used in the context of a line of descent.

For example, a family tree may show a rectus line of descent from one generation to the next, indicating a direct line of ancestry without any branches or deviations.