What does el5, el6, and el7 mean?
EL is short for Red Hat Enterprise Linux (EL).
EL6 is the download for Red Hat 6.x, CentOS 6.x, and CloudLinux 6.x.
EL5 is the download for Red Hat 5.x, CentOS 5.x, CloudLinux 5.x.
EL7 is the download for Red Hat 7.x, CentOS 7.x, and CloudLinux 7.x.
The UNIXy Varnish Plugins run on all the above platforms.