You're right, "uncompressed" is basically the raw audio in digital form, often called WAV or PCM or LPCM. It's the same as the studio master, say 5.1 channels of 24-bit audio sampled at 48 KHz = 6.9 Mbps. "Lossless" means this audio is compressed mathematically, so that when it's uncompressed it is exactly the same the studio master -- like a ZIP file can be compressed and then uncompressed without losing anything. TrueHD and DTS-HD MA are lossless formats.
Dolby Digital and DTS are "lossy" formats, which means that they are compressed not only mathematically but also by throwing away parts of the audio you're not likely to notice. So they're not the same as the studio master, but the higher the bitrate, the less likely you are to be able to tell the difference -- like using more or less compression with a JPEG. You've already worked out the limits -- 640kbps for Dolby Digital and 1536kbps for DTS -- though that's not per channel, it's for all the channels in aggregate.
Any multichannel format higher than Dolby Digital and DTS (e.g. DD+, TrueHD, DTS-HD) need to be carried over HDMI. If you're playing them over optical then only the legacy Dolby Digital or DTS track that usually accompanies them will be played.