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recommendations - favourite category of the user
How to make product recommendations with user favourite category using category affinity?
First step is to ensure you're setting the category value(s) in the category parameter.
For Target Standard (no access to Target recommendation) you create audiences for each category you wish to personalize/promote certain products:
For Target Premium where you have access to Recommendation there's a good handful of out-of-the-box criteria that allows you to promote products based on the favorite category for the individual user:
They are also great for getting inspiration if you want to create your own.
If valuable, I have this 'demo' site on how category affinity works: https://accrease.com/adobetarget/category-affinity.html