In the Embed URL Builder, this is the Email field.
Syntax
Properties
A valid email address for the embed user.
Examples
Effect on email-only users
When this parameter is set, Omni checks for any existing email-only users with a matching email. If a match is found, Omni automatically transfers the email-only user’s schedule recipients to the embed user and removes the email-only membership. This happens on embed login.
After the merge, personalized deliveries use the embed user’s attributes instead of the email-only user’s defaults — which is important when access filters are in use.
Set this parameter for any embed user who is also a delivery recipient. Without it, Omni creates a separate email-only user for that address with default attribute values. 
