libexec directory?

Hi, I noticed that a relatively small number of MacPorts (but some of them being quite important such as slapd for OpenLDAP and mysqld) install binary executables into the {prefix}/libexec directory compared to the much more often used and larger {prefix}/bin directory. I'm curious what is the logic for having two separate directories one named "libexec" and the other named "bin"? Thank you, -T.M.