Note that the option "Override Allowed" controls whether your user sees a context field in the flexfield pop-up window. You set "Override Allowed" to Yes if you want a context field to appear in the descriptive flexfield pop-up window. You set "Override Allowed" to No if you do not want users to choose a structure from within the pop-up window.
In these diagrams, "OK" means that whether you specify Yes or No for an option does not matter (another option may have an "overriding" effect). For example, if you have a default context field value (structure choice), but you have a context field as well, your default value will appear in the context field but the user can choose a different value instead.
Another way to achieve a similar effect is to define a single structure that contains only context-sensitive segments. You also define a default context value, and you do not provide a context field or a reference field. The effect of this setup is that the user always sees the same segment structure, so it behaves as if it were a structure of global segments. However, if later you needed to add more structures of context-sensitive segments, you could do so by enabling the context field or a reference field, disabling the default context field value, and defining your new context-sensitive segment structure. Note that if you had already used all the available segment columns in your first context-sensitive structure, you would not be able to add more segments to that structure; you would only be able to define additional structures. One drawback to using the context-sensitive segments only strategy is that if you have certain segments that should appear for all contexts (all structures), you would have to define those segments separately for each context-sensitive structure.
Of course, you could initially define a hybrid structure that contains some global segments and some context-sensitive segments but has only one context-sensitive structure with a default context field value (but no context field or reference field).
How Segments Use Underlying Columns
Planning Your Descriptive Flexfield
Descriptive Flexfield Structure Planning Diagrams