When Luminate attempts to match an action with a constituent who is not logged in, such as when they submit a survey or donation, it will search both primary and secondary email fields if the secondary email is enabled.  If the email entered matches either the primary or secondary email of a record it will be able to match the constituent. This means that the interaction the constituent took will show up on their record and no new constituent record will be created.

Note: This was tested with unique email enforcement for registration, specific behavior may be different if you do not enforce the unique email requirement for normal registration.