That's because there isn't a definitive answer under the Washington statutes, court rules, or case law. In practice, courts will view it as going toward the basic expenses every child needs: shelter, utilities, food. Clothes are arguable, but IMO clothes are included too.
Whether you have to contribute to expenses above the monthly support payment depends on section 3.15 of your support order. In addition, some expenses are included by law to be apportioned between the parents according to income. See RCW 26.19.080 at this link:
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