Use Class is a term taken from BS EN 335:2013 (Durability of wood and wood-based products. Use classes: definitions, application to solid wood and wood-based products). This standard groups possible end uses for timber into 5 classes referred to as Use Classes.
Use Class 3 relates to timber used in above ground, outdoor situations and therefore this is the category that corresponds to timber cladding.
Timber species with insufficient natural durability to be used successfully outdoors can be upgraded by the application of a factory-applied timber preservative to improve its long-term performance. The application of preservative is not a ‘one size fits all’ process; it is tailored specifically to the intended end use of the timber product.
BS8417 Preservation of wood – Code of practice and the Wood Protection Association's Code of Practice: Industrial Wood Preservation are the relevant standards for wood preservation in the UK. They define treatment by preservative type, penetration and retention i.e. amount in the wood relative to species and desired service life (15, 30 or 60).