Difference between -1 and null in the targettype filed

Hi

Could anyone advise what is the difference between -1 and null in the targettype filed in marketing documents?

For example, in Sales Quotation - Rows (QUT1) table, I found for some open sales quotations, the field is null however the DB help file suggest the default value is -1 so I suppose it should be -1 for open documents.

Many thanks for your advisory.