Back to the difference between qualification and certification.
Generally, the third party (you) witness the welding of the coupon and then evaluate the coupon by visual, volumetric NDE, bend tests, etc. The result of the evaluation is recorded on the performance test record. The third party signs the test record. I sign the test records I issue as the test witness and I sign for the tests that I performed. With that, the welder has been qualified. However, as the qualifier, I have no idea what welding the welder will be performing, what materials, what position will he or she be welding in, etc. In short, I do not assign the welder to any particular job and I have no control over what the welder does.
At the bottom of the performance report is a place the welder's employer signs attesting that the report is correct and true. That is the "certification", i.e., the employer's signature. The codes make the employer responsible for ensuring the welders in his employ are properly qualified for the work, i.e., the employer signs the performance test record. The employer is responsible for assigning the welder to jobs that his is appropriately qualified to do. It is the employer's responsibility to know what the welder is qualified to weld. It is the employer that is liable for the welder's work. Should a weld fail for any number of reasons, the employer bears legal responsibility and liability.
I often wonder if people "certifying" the welder fully recognize the potential liability involved.
The CWI credential is a certification that the individual is qualified to perform visual examinations of welds. It does not limit the CWI to a particular code. It is the employer's responsibility to ensure the CWI has the experience to use the particular welding standard or to perform the assigned work. The CWI has an ethical obligation to take on tasks that he is capable of performing. If the CWI is uncomfortable with the assignment, he should inform the employer and defer the work to someone else. That's the way it is suppose to work. In the real world the CWI does what he is directed to do. Just remember, if anything goes wrong the employer will throw the CWI under the bus and never look back. It is the CWI that bears the possibility of answering to that Certification Committee if charges are initiated because the CWI took on responsibilities for which he or she is not qualified to fulfill.
Good luck. The other posts also offer sound advice.
Al