What is a "Null Key"?
A "null key" is a key where not all records are put in the index, and thus these records are not found when using that key. When using SeekBy methods, this basically means that you get an automatic limiting of records.
Take the following code example where we iterate over the "entire" Sales object using the SeekByUnpaid method:
bool more = ObjSales.SeekByUnPaid(smFirst, LowValue_Date);
while (more)

    more = ObjSales.GetNext();
This code does not appear to perform any limiting or filtering. And yet, it will only fetch records where ObjSales.pIsTicked is false. The key has been defined such that records where ObjSales.pIsTicked is true are not put in the index.