SQL Server Option for Navision

What happens if we have NULL in SQL database? How Navision for SQL Option understand it,like default values or not?

C/SIDE will interpret the NULL as the default value for that field type - 0, ‘’, 0D, etc. But it will never write a NULL back to the field if you update the record. It will always write back the default value, so it will no longer be NULL in the field; i.e. it is not symetrical.

Thank you very much for your answer.