After creating a lookup relation on an MDA Data - Object in Rules Engine you can select the MDA Data - Object to run a rule.
Note: Before performing MDA Joins in Rules Engine, you must create a lookup relation on an MDA Data - Object.
IMPORTANT: MDA joins are supported in Custom and Bionic Rules.
The following image highlights how various collections such as Account and Associated User are linked to Joins usage.
Note: You are provided with only 3 sub-level objects containing lookup relation. For example, in the following image Joins Usage -> Account -> Active User -> CSM, where Joins Usage has a lookup relation on Account, and Account has a lookup relation on Active User which in turn has a lookup relation to CSM.
Sample Use Case
You might want to fetch Account and Count of high priority cases generated over the last 7 days for specific industries.
In this rule on MDA, Account has a lookup relation with Case.