The common parameters listed in Common Parameters are always required. Most service calls have additional parameters, and most of them are required, but a few parameters are listed as optional. An optional parameter may be shown as omitted in one of three ways:

• If no parameters follow it, by passing a short parameter list. As noted earlier, all parameter lists are to be constructed in the VL or varying length form.
• Providing binary zero in place of an address in the parameter list.
• By passing a null value. The null value for a varying length character string such as SECTION, is a zero-length string. The null value for fullword or doubleword parameters is a fullword or doubleword of binary zero.