You are right, I have never seen a description of what the difference between those two setting means.
My guess is, these settings set other settings that are optimal for either a SSD or HDD (wow, big surprise, right?)
For example, older mother boards would have a setting for HDDs, that was like a timer you could set, from say 30 seconds to 0 seconds. Whatever amount of seconds you chose would delay POST while the HDD(s) started up. HDDs take longer to start up than SSDs, and older HDDs took longer than newer HDDs.
If you use a SSD for your OS drive, which should be on SATA port 0 or 1 (SATA III ports), there is no need for a delay to wait for the SSD to start up. That would make the POST time shorter, and the PC will boot faster.
There could be other settings that are better for SSDs or HDDs that choosing SSD or HDD for that option is preferred.