Distinct and Allow Nulls & what do they mean on the data sources tab?

Distinct:

When selected, if 2 or more records are identical across every field, show it only once. Otherwise it will be shown once for every time it appears in the data set, which due to the joining of dataset could be many times.

Allow Nulls:

The technical explanation of this is when it is selected the join between the data sources is an outer join, as opposed to an inner join when unchecked. What this mean is that if it is unchecked, any record that is not represented in EVERY data source used will be excluded from the report data set. When checked every record will be represented but all values in the data source(s) where it is not represented will be blank or NULL