What is the standee line, and where is it located on a bus designed to allow standing?

49 CFR 383.117(a)(6) (operating practices not otherwise specified); AAMVA CDL Manual Section 4.2.3 "Standee Line," p. 4-2

Standing riders too close to the driver can block the driver's view and interfere with vehicle control, which is exactly the safety problem the standee line exists to prevent. The exam expects applicants to know both what the line is for and where it sits on the bus.