About putting custom fields in a folder

The ID (custom_fields) of folder that the custom field is add into is programmed into the IssueTrackerProduct. In the IssueTrackerProduct's Add Issue page it's hardwired to look for this folder and if found it will automatically include it as part of the Add Issue page. This folder is also ordered which means that you can control the sort order that the custom fields should appear which might have an impact on the user experience.

The alternative is to create the custom field on its own outside a custom folder but then they have to be added directly inside the IssueTrackerProduct instance and you can't contol the order.

An added advantage of putting all custom fields in a folder is they can be acquired via Zope's acquisition. This means that if you have two issuetrackers where you want both to use your custom fields you can create the custom fields in the trackers' parent folder and all issuetrackers will be able to use the custom fields.