Symbolic parameter means if you write a one catlog procedure in that you have given a
//dda dd dsn=somename.
if you are calling the catlog procedure each time the data set name is fixed.But you want to change that data set name when ever you want the you can recode that step like this
//dda dd dsn=&s
if you are using that catalog procedure in some ware another job
//step1 exec=Ctalog procedure name s=abc
this value is repalced in above statement.
//dda dd dsn=abc
The symbolic parameter is used to override parameters on dd statements.They can be used in both cataloged and instram procedures.provided a convenient means of assigning values to commonly used jcl parameters.Are assigned values in the Proc or EXeC statements.