The <AttributeResolver> element configures components that can be enabled to obtain additional attributes about the logged-in user following a SSO event, as well as transforming or creating new attributes internally.
During SSO, the IdP can (and generally does) supply attributes in a "push" fashion inside the SAML assertions it issues. These attributes are decoded with an AttributeExtractor and cached with the user's session. The purpose of a resolver plugin is to "pull" attributes from additional sources or to transform existing attributes in some way.
Like most plugins, the type attribute determines which type of plugin to use. Each type supports its own attributes and child elements.