The importance of parameter order, and using placeholders for optional parameters

For several of the ISPF services, the syntax for call invocation (shown under "Call invocation format") includes a number of optional parameters. These optional parameters are positional; that is:

Each optional parameter you choose to specify must be specified in the sequence shown in the syntax diagram.
Where you specify a parameter without specifying one or more previous optional parameters, the absence of each previous optional parameter must be indicated by a placeholder in the form of a blank enclosed in single quotes followed by a comma. (For the purpose of clarity, the syntax diagrams show only the comma.)
For example, in the call invocation for the ADDPOP service shown here, the first and third optional parameters have been specified (field-name and column respectively), whilst the second optional parameter (in this case, row) has been omitted:
```
CALL ISPLINK ('ADDPOP  ', field-name, ' ', column);
```
When you do not specify an optional parameter, the default value (if any) for that parameter applies.

It is only necessary to include placeholders for unspecified intermediate parameters. That is, once you have specified the last optional parameter you want, there is no need to specify placeholders for subsequent optional parameters (if any); if there are default values for any subsequent optional parameters, then they will apply.

Further examples:

All optional parameters specified:: CALL ISPLINK ('DISPLAY ', panel-name, message-id, cursor-field-name, cursor-position, stack-buffer-name, ret-buffer-name, ret-length-name, message-field-name);
First five optional parameters specified:: CALL ISPLINK ('DISPLAY ', panel-name, message-id, cursor-field-name, cursor-position, stack-buffer-name);
First, third, and fourth optional parameters specified:: CALL ISPLINK ('DISPLAY ', panel-name, ' ', cursor-field-name, cursor-position);
Third, fourth, and seventh optional parameters specified:: CALL ISPLINK ('DISPLAY ', ' ', ' ', cursor-field-name, cursor-position, ' ', ' ', ret-length-name);