Once you have created a customization for your SME Server by adding new files, directories, and symbolic links (for your actions, events, etc.) - and perhaps also triggering an action to initialize your customization - you are ready to package your customization into an RPM.
All SME Server software packages are distributed as RPM packages. This is the format used by CentOS and other major Linux distributions for distributing applications and other collections of files. The RPM system provides the ability to install, upgrade, remove and (importantly) verify the contents of installed packages.
An RPM essentially consists of an archive of all the files required by a piece of software. Additionally, it includes meta-information describing the software, and scripts which must be run to install or uninstall the software.
Meta-information stored in an RPM includes:
summary and description of the software
category/group to which the software belongs
name and email address of the packager
pre-requisites to installing this package
... and more