A more thorough explanation:

Definition: Senescallus (sen-uh-skal-uhs) is a term from Law Latin that means the same as seneschal.

Seneschal: Seneschal (sen-uh-shuhl) is a term from Law French that has several meanings:

1. A French title of office, equivalent to a steward in England.
2. The steward of a manor.
3. An administrative or judicial officer, such as the governor of a city or province.

For example, a seneschal could be the person in charge of managing a noble's estate or the governor of a city.

The term senescallus is simply another way of referring to a seneschal, and it is used interchangeably with the term in legal contexts.