I received a response from my ticket and this feature is explained as such:
"This configuration you are referring to in BoxTrapper is specifically for forwarders that are created on your cPanel server here, and not forwarders from a remote server.
A forwarded message that is from a remote server is processed just as any other SMTP transaction from a remote source. While the email content may indicate that the message was forwarded, this is irrelevant to the actual delivery process as the connection must still go through all validation processes. Your server is not inherently aware that the message being delivered is a forward, nor does it need to so long as the remote server passes validation during the connection. As such, these remote messages forwarded to your server here are processed via BoxTrapper when the account has it enabled.
Instead, this configuration is to ensure that any "Forwarders" that you create in your cPanel account on this domain are processed correctly with a BoxTrapper-enabled account."
Here is an example the tech provided: (I replaced my email addresses and replaced them with examples)
For example: if you created a forwarder named "
[email protected]" which forwards messages to "
[email protected]" in your cPanel, then you will want to ensure that "
[email protected]" is defined in your BoxTrapper configuration for "
[email protected]". This is to ensure that messages sent to "
[email protected] are handled correctly when being processed by BoxTrapper, as local alias delivery takes a different router in Exim than normal SMTP transactions.