post_id: Post ID.
taxonomy: Taxonomy Slug.
append: If false will delete difference of terms.
terms: comma-separated or array of terms.
slugs: comma-separated or array of the slugs.
Relates an object (post, link etc) to a term and taxonomy type. Creates the term and taxonomy relationship if it doesn’t already exist. Creates a term if it doesn’t exist (using the slug).
A relationship means that the term is grouped in or belongs to the taxonomy. A term has no meaning until it is given context by defining which taxonomy it exists under.