The IsBlank(expression) function takes a field or an expression and return 1 (or true) if that field is blank or null or if the expression evaluates to a blank or null value.
Example:
IsBlank("Fax No.") ---> 1 (true) if the field "Fax No." is blank or null, otherwise 0 (false)
IsBlank(DBLookup('DESTDS','User','Alias',"Salesperson",'Id')) ---> 1 (true) if the DBLookup function does not find a User for this Salesperson, otherwise 0 (false)
The formula
##IF(EQUALS(TAXGROUP,''),0,1)
can now be written like
##IF(ISBLANK(TAXGROUP),0,1) or even shorter like ##1-ISBLANK(TAXGROUP)