Issue Collector: how are the custom fields in the collector form chosen?

[StephanieC]

Issue Collector: how are the custom fields in the collector form chosen? We need to expose fields that do not show as available when configuring the issue collector form - custom option. Most of what is listed are internal fields and we need data from external client input.

Are they specific custom field types? Can I add fields to this list (or subtract them)? 

Also, is there a way to default this checkbox to "checked": Issue Collector: how are the custom fields in the collector form chosen?