The ScheduleDefinition class contains various settings that define the contents of a schedule view, including:

• The schedule's category and other basic properties that determine the type of schedule.
• A set of fields that become the columns of the schedule.
• Filters that restrict the set of elements visible in the schedule.
• Sorting and grouping criteria.

Most schedules contain a single ScheduleDefinition. In Revit MEP, schedules of certain categories can contain an "embedded schedule" containing elements associated with the elements in the primary schedule, for example a room schedule showing the elements inside each room or a duct system schedule showing the elements associated with each system. An embedded schedule has its own category, fields, filters, etc. Those settings are stored in a second ScheduleDefinition object.