Symbian OS v9.X SIS File Format Specification

This paper, written by Symbian's Security team, describes the newly restructured and redesigned SIS file format which is introduced to complement the new security architecture of Symbian OS v9.1. This new format is sometimes called SISX. SIS files are used as the primary means of packaging files for deployment to a device, and are interpreted accordingly by the software installer. As well as simply containing files, the SIS file can contain conditional statements which influence the installation e.g. device specific installations, language-specific installations, and user-selectable optional components.

You can grab the document from Symbian Developer web site