For the current documentation please see the doc/readme.md
check_multi is kind of a wrapper plugin which takes benefit of the Nagios 3.x capability to display multiple lines of plugin output. It calls multiple child plugins and displays their output in the long_plugin_output. A summary is given in the standard plugin output.
Normally the child return code with the highest severity becomes the parent (check_multi) plugin return code. But you can influence this by specifying flexible state evaluation rules.
The configuration is very simple: a NRPE-stylish config file contains a tag for each child plugin and then the check command line.
Questions and comments are always welcome at (Matthias.Flacke-at-gmx.de)