The basic attire a survivor begins with when they begin a new character.
It cannot be looted from dead players or found in loot piles. To obtain a new set of clothing, it must be picked up from where another player has switched it out for another set of clothing.
The clothing appears in different colours and is client side. This means that one player could see it as black and white, whereas the other player sees it as blue and beige.
If you do not have the Private Military Company DLC, then the clothing will appear in a low resolution version.
If people already have civilian clothing and dress up in other civilian clothing, they will appear with black clothing (also looking black for all other players).