What is 'Trusted Keys'?

[Jeff Cai <Jeff Cai Sun COM>]

Hi, list

What is the meaning of "Trusted Keys" in Seahorse? These keys only
include PGP Keys? If yes, I think how could we let user know that SSH
keys can't become trusted keys?

Additionally, Could we get other keys besides PGP keys from key servers?
I'm confused about which is only meaningful for PGP keys and which is
meaningful for both.

Jeff