For preparatory purposes before importing your file with subscribers and subscriber data, we recommend that you create the custom fields in advance within Rule. This way, the matching of the fields will be as smooth as possible when you import your file.

It is important to note that the custom field for email address does not need to be created manually.Rule reads the @ address during the import and creates the field automatically, naming it "Email". See screenshot below.

This is what it looks like when the field for email address is automatically matched: