Plank is refering to a comcept of a unit of planks, instead of individual planks. This is done to keep numbers down to an easily manageable size, as bloated numbers don't add intrinsic value. That's why multiple Logs can turn into a single unit of Plank.
Crafting cost:
Sawmill cost:
This resource is renewable and can be made in a cheaper way by a specialized building (Sawmill) than by simply crafting in a Workshop or Manufacture.