I'll take you down the only road I've ever been down

[venta]

I have to make a birthday cake for this weekend... and I was considering making the port and chocolate cake from Cooking for Geeks.

However, the recipe requires "bittersweet chocolate", and has a note not to substitute "semisweet chocolate" due to their differing cocoa fat content.

I'm not sure that I completely understand how those USian folks use the terms bittersweet and semisweet. Can anyone translate for me? Wikipedia's classification of chocolate describes the two kinds as "interchangeable when baking"...

I'm low on time this week, so no chance of solving this experimentally.