If you need to create your own parameter id which can be bound to a data element or which you can use in your abap commands SET/GET PARAMETER ID, you can follow the steps described below.

One simple way how to create a new SET/GET parameter ID is to go to Tx SE80, select category Package in the listbox, right-click on the package and select Create -> Other (1) -> SET/GET Parameter ID

What it actually does is adding a new SET/GET Parameter ID to the table TPARA. If you are not friend of SE80, you can maintain table TPARA directly in Tx SM30.

Now if you want to assign a default value for this parameter to a user, just go to SU01 / SU3, go to tab Parameters and add you freshly created parameter to your list of parameters with a meaningful value (this value is limited to 40 characters)

This parameter ID can also be

• bound to a data element
  
• Used on a selection screen (with addition of MEMORY ID) so the selection screen parameter is(can be) automatically pre-filled with value from your user defaults (Tx SU3) or by value set previously (e.g. in another program) by the following ABAP command.

  SET PARAMETER ID [param_ID_name] FIELD [variable_to_store_param_ID_value]