So, when I hit my trigger key (in my case the "~"), the above menu pops up and I can hit "c" (for instance) if I want to forward it to Heap and have it automatically be placed in the category "Custom Software." Or I can hit "s" if the e-mail is a support issue, etc.
Remember, Heap will automatically associate it to the correct lead, opportunity, customer or archive, and none of this categorization does anything to change that.
So, how does this work? Let's take a gander at my mail rules:
Ok, Act-On works on the concept of defining a letter then a "|" followed by a description. I would recommend you use the sample entries and just duplicate them. Let's edit the "Forward to Heap (Category:Custom Software)" rule.
Alright, you see from the perform menu, I've selected "Forward Message" and I've told it to forward to email@example.com. Remember to substitute your Heap sub-domain where I have "wbp". But if I just do that, the message will get into Heap and it will auto associate with the correct lead, opportunity, customer and archive; but it won't be placed in a category. The fix: click on the "Message..." button.
Ok, as you can I placed a little note in here that will automatically prepend to the message. Again, in your case put the name of the category you want to put the message in, in place of "custom software". Also, make sure that you create one of these rules without any message text, so you can tell Heap to do the default action: place the message in no category at all.