What to enter in the PayPal Merchant Account ID field

Hello. I am using Membership 3.5.1.1 and the PayPal Standard Payment Gateway.

In the Membership > Payment Gateway, in the Settings for the PayPal Standard Payment Gateway, what should I enter for the Merchant Account ID? I read somewhere to use your PayPal business account's primary email address.

But when I log into my PayPal business account, and go to Profile > My Business Info, I see there is an alphanumeric value for the Merchant Account ID.

So should I use my PayPal business account's primary email address, or the alphanumeric value I see in my actual PayPal account?

Thank you.