In real estate, what do people often mean by' case field'?

It means scene, also called sales case field, which means the scene of sales office.

For example, case sales, commonly known as sales clerks, are sales gentlemen or sales ladies in the sales offices of newly-built commercial houses.

The security guards at the crime scene are generally security guards recruited by property companies or buildings, and are responsible for the security work of a residential area or property jurisdiction.