Apart from the installation configuration template/update (for details see Configuration Template/Update), an ECS Package can contain other various files, such as xsl stylesheets, xml schemas, xml catalogs, Serna view files, etc. The files serve for a company's document type presentation in Serna and for Serna GUI customization.
To learn more about them and to get instructions on how to create them, see Document Template Development, XSL Stylesheet Development, ContentMap Stylesheet Development, Creating Element Help, and Serna View Customization.