The identity resolution add-on is useful when:
you have several data sources for your clients and prospects
you don't have a common identifier between these data sources
In a few words, to merge together two users coming from different sources into a single identity, we need to set:
The identity resolution rule: one or several fields used to define if two users can be merged together into one (email address, phone number...)
The user profile resolution rules: define which values to keep when two users are merged together. You can choose to prioritize a source over another, to keep the oldest or the latest record, to group several fields from one source together...