gnome-keyring-daemon components and their use

When reading gnome-keyring-daemon manual, one can see pretty clearly that the --components option has four valid values: ssh, secrets, gpg, and pkcs11. However, I couldn’t find any detailed explanation on these four options. Could someone detail the **use** and the **field of use** of each component of gnome-keyring-daemon? (For instance it is obvious to me the ssh component is used to automatically decrypt ssh private-keys passphrases and feed them to ssh but other components (especially their field of use) remain obscure to me.)