Domestic D is also generally a x-UP ticket where it is discounted first class as well, similar to the P fare... but the D is because the flight continues on to intl as you noticed.
You can see your fare basis if you look at your itinerary and go down to the bottom and click on the receipt link, or in the email with your flight confirmation. It will show something like HA00AUPfor example in the fare basis.

The info about possible being stuck in coach if the flight is canceled etc is true, however the agents should try to accommodate you into F on the next flight... however the next flight with first class seats available may be much later. Same is true even if you have a full F fare... if your flight is canceled and there are no seats available later, you have your choice of sitting in coach or waiting for a flight with F open.