The SNMP supports operations on MIB objects whose syntax is ObjectSyntax as defined in the SMI. Informally stated, SNMP operations apply exclusively to scalar objects. However, it is convenient for developers of management applications to impose imaginary, tabular structures on the ordered collection of objects that constitute the MIB. Each such conceptual table contains zero or more rows, and each row may contain one or more scalar objects, termed columnar objects. Historically, this conceptualization has been formalized by using the OBJECT-TYPE macro to define both an object which corresponds to a table and an object which corresponds to a row in that table. (The ACCESS clause for such objects is "not-accessible", of course.) However, it must be emphasized that, at the protocol level, relationships among columnar objects in the same row is a matter of convention, not of protocol.
Note that there are good reasons why the tabular structure is not a matter of protocol. Consider the operation of the SNMP Get-Next-PDU acting on the last columnar object of an instance of a conceptual row; it returns the next column of the first conceptual row or the first object instance occurring after the table. In contrast, if the rows were a matter of protocol, then it would instead return an error. By not returning an error, a single PDU exchange informs the manager that not only has the end of the conceptual row/table been reached, but also provides information on the next object instance, thereby increasing the information density of the PDU exchange.