Lookups can be used to fetch some additional data based on values in original data.
There is a tiny csv file that maps "article" to "This was 'article'. You can find it here.
The actual lookup is done with collection lookup node: Process data > Lookup data > Field: Look up value from collection
You need to add a new collection to your project and import lookup data there. Then, in the original collection, add a the collection lookup node.
Annif is a service for automatic subject indexing for text written in english, finnish and swedish.