Currently, when adding a new custom field in Jira you first create the custom field context and then associate the field with the applicable screens.

After setting the custom field context, as demonstrated below, the list of screens to associate the field to is not specific to the context that was set for the field.

The improvement request is to filter the screens listed in the "Associate field <Custom field name> to screens" menu to only show the screens associated with the project and/or issue types set in the custom field context.