Birck-bark clothing is derived from this material.
Real life context
URW Game Encyclopedia description
The official URW in-game encyclopedia (accessed by the key) describes birch bark thusly:
When birch-bark is used in clothing its use is restricted to shoes, head-dresses and small pieces for spot protection or to be used as ornaments etc. This is because finished birch-bark equipment isn't very elastic and feels coarse against the skin. More commonly birch-bark is used in making of various artifacts, knapsacks for example. Birch-bark garments give some protection against edge and tear attacks but for point or blunt strikes they are somewhat useless.
Birch-bark clothing articles and other birch-bark items in the game.
In-game encyclopedia entry on birch bark material