The North American meaning
In North America "stringer" usually means the long beams of a stringer pallet, where there are no blocks at all. On a block pallet like the EPAL 1, the word refers to the boards that connect the blocks. Be clear which you mean when talking to a buyer.
EPAL's rules
EPAL's EPAL 1 page says boards are 22 to 24 mm thick with stringers up to 25 mm, that poplar is not permitted for stringers, and that stringers must have no wane, the rounded edge where bark used to be.
Why stringers matter for grading
Stringers carry the deck. A broken or split stringer usually means a broken board under the classification, which puts the pallet in not fit for use until a licensed repair.