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.


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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.

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