How does SQL define which is a NULL and which is a Blank?

Hi All,

I have a table which loads the data from an excel file and i have 2 columns in the excel file which are blank one is datetime and the other one is varchar. When i load this excel into the table the datetime field shows NULL for blank space and the varchar column shows blank. I am just curious to know how does SQL define this? Is there anyway which says this should be NULL vs this should be a blank? FYI both of my columns are NULLABLE .

Thanks to everyone who provides their inputs on this.

Thanks