The fields that NetSuite makes available in Formstack Sync are found on the Default forms, found in the Customization -> Forms -> Entry Forms Menu
On the list that is shown, only the fields that are on the "Preferred" forms for each type will be used. There can only be one that is "Preferred" for each object type. The ones that are relevant for Formstack Sync are: customer/lead/prospect, contact, opportunity, supportcase, employee, and calendar event. Any of the standard forms can be edited and marked as preferred.
Within a form, a field that is marked as "Mandatory" will give a MISSING FIELDS message in Formstack Sync if it is not filled out. If you change any "Preferred" or "Mandatory" settings, you must click "Refresh Field Info" back in Formstack Sync in order to update our schema.
By default, some fields in NetSuite refer to objects that are not accessible by the objects that we do sync. Fields that are calculated based on objects not listed above (in bold) may still be accessible within the other forms. There is a permission available called 'Run Unrestricted' on the Results tab of the Saved Search that NetSuite is sources from that can allow accurate results for these fields even if users don't have permissions to see the underlying records in some cases.