Use fieldsets to query fields on existing records
B25.FormRecords (such as returned by B25.Form.getActiveRecord or B25.FormRecordCollection.get, for example) for existing records get their fields populated by GoMeddo queries before your CFL is executed. GoMeddo populates all the fields contained in any fieldset on the target object. This means that if your CFL implementation references fields that are not present on the form, you can let GoMeddo populate them by adding any fieldset containing those fields.
For example, your form displays Service Reservations, and your CFL has a handler that uses a checkbox on Service Reservations to decide what to do with them, but you do not want to display this checkbox on the form. When you test your CFL on an existing Reservation with existing Service Reservations, it fails because the checkbox has not been populated and always contains null. One option available to you is to modify your CFL to do your own query to get the values in this field. However, this is bad for performance (Service Reservations now get queried twice) and unnecessary. The better option is to simply add a fieldset on Service Reservation containing the checkbox field, and presto, the field is now populated.