Assoc Types define the type of a relation two items have.
For example: ITEM1 - VIEWED_TOGETHER - ITEM2
Typical examples for such Assoc Types are
VIEWED_TOGETHER
BOUGHT_TOGETHER
GOOD_RATED_TOGETHER
IS_RELATED
Additional assoc types can be created in the type manager within the easyrec web application admin gui. Recommendation algorithms (or rule generators/plugins, as we call them) are associated to assoc types. This means, if you want to use additional generators, you have to create assoc types so that you can associate a plugin configuration to it and query the results using the API (e.g. the relateditems call).