For string columns and variables, the empty string is an often-used synonym for the zero-length string: ''.

I guess a varbinary could also be considered empty when the contents are zero-length. Other data types do not support an empty value, as there are no empty values in the various numeric domains, nor in the date, time, or datetime domains. (Maybe xml does support some kind of empty value, though I think you can only do that with untyped xml - but I am far from an expert in the field of xml, so I might be wrong).