Yes, Form is designed for a plugin and play setup with very little configuration as possible by the end user. It’s not a form builder.
The dev doc is inline phpDoc. For devs that want to build their own custom fields I refer them to the free extensions which utilize them.
These use builtin supported field types and are fairly simple to implement:
- http://wordpress.org/plugins/connections-business-directory-income-levels/
- http://wordpress.org/plugins/connections-business-directory-education-levels/
- http://wordpress.org/plugins/connections-business-directory-languages/
This extension uses a custom field type to store and save the business hours. It is a bit more complicated but shows what can be down.
Here’s a link to an inline phpDoc example but referring to the actual code in the free extensions should be easier to understand:
And here’s a link to the supported field type in the core plugin:
If none of those built in field types work for you, you can add a custom field type like it was done in the Business Hours extension.
Hope that helps!