Definitions for "Null value"
Keywords:  blank, zero, absence, column, anevent
A parameter position for which no value is specified. IBM.
Having no explicitly assigned value. In particular, a NULL value is different from a zero or a blank.
Absence of a value in a column of a row. Nulls indicate missing, unknown, or inapplicable data. A null should not be used to imply any other value, such as zero.
Keywords:  wildcard, matching, service, types
a wildcard, matching all services
a wildcard, matching all service types
an empty string with no nul-terminator
a string with no characters and is represented by quotation marks (" ")