Check out the Knowledge Base article on classes, methods and properties: https://www.dotcom-monitor.com/wiki/everystep/everystepscripting-tool-classes-methods-properties-etc/
Beyond these documented elements, basically script is a valid C# code run in a sandbox. For the interaction with websites it uses a set of classes with predefined methods and properties, which work directly with HTML.
RIA feature works with browser window directly, without considering page origin (HTML, Flash, PDF, etc.)
Some further info/samples can be found here: