In the current release (4.15), custom fields are configured in the Custom Fields table of the NetSuite snaps. In the new release (4.16), custom fields appear in the snap’s input schema and can be mapped just like standard fields. Furthermore, 4.16 has more complete and correct support for custom fields across most NetSuite object types that can have custom fields.
Please share the specifics of what you’re trying to do: the Object type selected in your snap, which “different objects” you’re referring to, etc. It will be much easier to help with specifics.