Variable symbols as points of substitution

Values can be substituted for variable symbols that appear in the name, operation, and operand fields of model statements; thus, variable symbols represent points of substitution. The three main types of variable symbol are:

Symbolic parameters (positional or keyword)
System variable symbols (see System variable symbols)
SET symbols (global-scope or local-scope SETA, SETB, or SETC symbols)

Examples:

&PARAM(3)
&SYSLIST(1,3)
&SYSLIST(2)
&SETA(10)
&SETC(15)

Symbols That Can Be Subscripted: Symbolic parameters, SET symbols, and the system variable symbols &SYSLIST and &SYSMAC, can all be subscripted. All remaining system variable symbols contain only one value.