This box should be activated for the channel that shouldn't overwrite other channel. Consider this example: let's say a user visits your site using Paid Search (the last touch channel is set to Paid Search), the user navigates to different internal pages (last touch channel will override to Internal) and then makes a purchase. In this case, "Internal" channel will get a credit for purchase which won't be useful for any business. So, we will leave the box unchecked for "Internal" channel.
Generally, it's recommended that we leave the box unchecked for both "Internal" and "Direct" channel.
Given your example (and use case), you can check for both "Direct" and "Internal", however, I'd still recommend to leave those unchecked. So, if a user would have earlier come through SEO channel and today Direct > Botón CTA (Internal) > Purchase, the SEO channel gets credit.