What Variables/Data is sent to SendGrid for email templates?

The Email notification system is working really well. I have a use case where my chat is being used on two different applications (like a marketplace, Sellers and Buyers).

Dependant upon a variable I want to show a different reply link within my email template so they get directed to the correct App.

At the moment we are utilising Tags to differentiate the users, we could also use the metadata.

Is there anyway to know what variables/data are sent through to SendGrid, so we can see what we can use within the email template?