When referring to Fedora versions, e.g. in package names, the version number is sometimes prefixed with fc. For example, a package built for Fedora 28 might have fc28 in its name. Similarly, CentOS packages seem to be named using el, e.g. el7 for CentOS 7.
What do fc and el come from, and how do they relate to Fedora and CentOS, respectively?