Pre-qualified means that if your joint, materials, processes, etc(conditions)...all meet the criteria spelled out in Clause 3 that you can write a WPS
without testing (pre-qualify)it via a PQR.
If your conditions do not meet the criteria, then you have go to Clause 4 and follow the instructions
to test (qualify)your set of conditions and if all passes, then you can write a WPS, but you must stay within the list of variables in Clause 4 for that WPS to be valid.
Reading the clauses as stated earlier is a good idea to become familiar with them.