ISO 21887:2007 defines five use classes that represent different service situations to which wood and wood-based products can be exposed all over the world. Subclasses are also defined for these use classes.

ISO 21887:2007 applies only to preservatives and processes for pre-treatment of wood and wood products and is not intended for products and processes for remediation and eradication of existing damage to timber. The annex to ISO 21887:2007 provides guidance on applying use classification to specify preservative treatments, treated wood, naturally durable and modified wood but does not provide definitive specifications.