Hi, What is the meaning of unmarked doors in the following text?

He collected his thermos of coffee, a cold biscuit with ham and carried it to an eighteen -wheel rig with unmarked doors and a ten-thousand-gallon tanker as its payload.



The doors of the truck had no company logo or signage on them.


A lot of big trucks have company names painted on the doors. This truck did not.


Students: We have free audio pronunciation exercises.

He is going to his semi-trailer truck. Such trucks often have prominent writing or logos on the doors to say what company owns them and the like. This one does not.

 AlpheccaStars's reply was promoted to an answer.
 Clive's reply was promoted to an answer.
Teachers: We supply a list of EFL job vacancies