afs:clientData stores random project-specific data, as specified during the PaF phase. It is used to add custom information to each reply in a Result Set.
For example, while indexing a product catalog, it may well be convenient to include the reference and price for each product in the index, and retrieve it in replies. This allows fast presentation of these items to the users without querying external sources.
Facet data can also be included in this section in order to display the facet values with each reply.
There is no semantic for the contents of this element, since it can include any data from the source document.
Words in clientData can be highlighted on queried word, see afs:store topic of the AFS v7.7 Configuration Guide for more information. Example for afs:query=Data: