stiff uniform

[KLPNO]

Hello everyone
I'd like to ask about the meaning of "stiff" in the following context:

Suddenly a short homely mockman entered the dining hall. It was female, dressed in the stiff uniform of a station worker.

Does stiff refer here to the fabric the uniform was made of, i.e. stiff fabric, or does it refer to the appearance of the uniform?

[Anglika]

My interpretation is that the uniform restricts movement - it is the material and design that are stiff.