AttributeDefintions produce a single IdPAttribute object, a neutral/internal representation of data. These objects eventually become SAML Attributes, OIDC claims, or other protocol-specific represenations through a process called "encoding". Encoding rules are established either through generic rules established via the AttributeRegistryConfiguration or, as in older versions, by attaching AttributeEncoder plugins within the Attribute Definition.
The ability to attach encoders is what distinguishes the product of an AttributeDefinition from an IdPAttribute produced by DataConnectors, so the difference has started to become somewhat historical with the new registry service providing a more generic facility.
Attribute definitions may also, but need not, be based on the output of a DataConnector, and often transform their input.
AttributeDefinition Plugin Types
An AttributeDefinition is defined using the (naturally) <AttributeDefinition> element, but each type of definition is distinguished by its "XML schema type", which is carried by the xsi:type XML attribute.
Copies an input attribute to an output attribute. Typically this was used to 'expose' attributes sourced from a DataConnector, and is often superfluous now with the ability to export attributes from them directly.