SAP ERP ==> SAP Basis

In transaction su01/ su3 we have a tab called as Parameters.  Why are parameters define?  How can we see what is meant by a particular Parameter (using Parameter id).

The parameters are the user parameters they will be in the structure USPARAM and in the table TPARA..

Parameters are maintained in the "Parameters" tab of transaction SU01.

Parameters has nothing to do with the authorization.
They are used to ease the daily life of a user.
For example if the user is responsible with one customer, s/he adds kunnr parameter here, to have this customer id filled when s/he calls the related transaction.

Parameter IDs (aka "user parameters") have nothing to do with authorizations; usually a user can even change the assigned parameters (respectively their values) using transaction SU3.

Parameter IDs are usually only used to memorize certain user-specific settings, e.g. whether you prefer the classic or the new ABAP editor / debugger, or some application-specific default values (e.g. the "customer number", KUNNR) which are used to fill-in corresponding entry fields (on selection screens / dynpros) automatically.

Please refrain from using Parameter IDs for any access control purposes.
 

So is it a good practice to manually assign parameter id's to the newly created users ?

Depends: if you want to clone an account then you should copy all attributes (including the user parameters); but normally you do not intend to copy all user-specific settings. That's at least what I would assume - but I live in the "ivory tower" and cannot comment on "best practices".

For Abapers:

Parameter id are SAP memory 

Application programs that use SAP memory must do so using SPA/GPA parameters (also known as SET/GET parameters). These parameters can be set either for a particular user or for a particular program using the SET PARAMETER statement. Other ABAP programs can then retrieve the set parameters using the GET PARAMETER statement. The most frequent use of SPA/GPA parameters is to fill input fields on screens 

set parameter id name field value.

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