The figure
22% is the ceiling EPAL's poster gives for new pallets. It is a characteristic of the class, not a promise that a given pallet is dry when it reaches you; a new pallet stored outdoors can take on water again.
Moisture against contamination
Surface moisture alone does not drop a used pallet out of classes A, B or C. Damp stains, mould and odour are different: EPAL lists them as contamination, which makes a pallet not fit for use.
Why it matters on long voyages
Damp timber in a closed container gives off water as temperatures swing, which can wet cartons and grow mould. Exporters shipping moisture-sensitive goods often specify new pallets or set a moisture limit in the purchase order and ask for a reading.
How it is measured
Usually with a handheld pin or pinless moisture meter. Readings vary by species and calibration, so agree the method with the buyer.