Comparison between dynamic fields and custom or extension fields

Dynamic fields are fields that are added to case and task templates to store additional information that is related to the case or task. Extension fields are custom fields that are stored in case record definitions or in extension record definitions.

Dynamic fields can be reused across cases and tasks, whereas custom or extension fields can be used in reporting, consoles, approvals, notifications, and service level agreements. 

Although both types of fields are defined in BMC Helix Business Workflows Catalog, dynamic fields are defined as questions, while extension fields are defined as process questions in questionnaires.