App Expert Reply:
A5 Documents app allows you to merge a related lists of records from the base object and you can also specify a filter (filter feature is detailed in this article). That filter can be a formula field that provides a true/false and that formula can provide any criteria you need. You cannot merge a list of records from an unrelated object.
If you have a specific use case, we suggest to create some object that can be your base object. For example it could be called “Report Document” and then you have some logic that relates records of different objects to this record to have the related records you need to merge. If you want to share your specific business with our support team, we can help confirm the best approach and assist you in configuring with app premium support as a service.