Exchangeable
The EPAL classification poster lists new, class A, class B and class C as exchangeable. Colour, light surface moisture and some splitting do not disqualify a pallet; the criteria are about structure, cleanliness and legible marks.
Not exchangeable
A pallet is not fit for use, and therefore not exchangeable, if a board or block is missing or broken, a block is twisted so it sticks out by more than about 1 cm, nail tips are visible, it is contaminated, it has been repaired outside the licensed system, or no EPAL mark is legible. Each of these has its own page under defects.
Why the line is drawn there
Exchange only works if both sides get the same value back. A clear list of disqualifying faults stops arguments at the dock, which is also why buyers in North America use the same vocabulary even though there is no exchange pool here.