A note about terminology: "Rigid" and "Flexible" refer to whether or not changes can be made to an existing Notebook (deleting, moving, sorting contents).  However, both types of Notebooks meet the 21 CRF 11 standards because all entries are time/date/user-stamped and audit trails are maintained that connect all versions.  The record of "who did what and when" is NOT lost when you choose to use Flexible Notebooks.

Your organization’s business policy determines whether or not you have the choice to create a Flexible Notebook. The chart compares some of the features of Flexible and Rigid Notebooks. Some of these features, in turn, are not enabled unless the business policy allows*.

If your organization allows flexible notebooks, and if users are permitted to create their own notebooks, you will have a choice at the time of notebook creation.  When you create a Notebook that is Flexible, this cannot be changed later.