To get access to this functionality you need to contact support. There is a limit of 10 elements that can be returned per bias (e.g. categories).
With Hello Retail you can personalize the listings of the categories for a customer. The benefit is to promote categories which a customer has an affinity to.
Below you can see an example of a customer using the API to show the top 6 categories using the user bias:
When using the user bias you can get the personalized hierarchies (categories) out and use it to decide what to show. Below is a short example of the code that can be used to get the hierarchy. The list of hierarchies returned in the get_bias is ordered by weight, which describes what is most popular.