In general, "cert's" that you may have, as issued by some 3rd party organization, are only good for backup information for your resume. For example, to be hired by a manufacturer/contractor to do production welding in accordance with ASME Section IX or AWS D1.1, you must be tested by that manufacturer. Previously held "certs" are not sufficient.
The following is directly from Article III, "Welder Performance Qualifications" of ASME Section IX:
QW-300.2
(a) The basic premises of responsibility in regard to welding are contained within QW-103 and QW-301.2. These paragraphs require that each manufacturer or contractor (an assembler or an installer is to be included within this premise) shall be responsible for conducting tests to qualify the performance of welders and welding operators in accordance with qualified Welding Procedure Specifications, which his organization employs in the construction of weldments built in accordance with the Code. The purpose of this requirement is to ensure that the manufacturer or contractor has determined that his welders and welding operators using his procedures are capable of developing the minimum requirements specified for an acceptable
weldment. This responsibility cannot be delegated to another organization.
AWS D1.1 does not describe this matter in as much depth, as does ASME Section IX, but says the following:
Para. 1.4.2 Contractor's Responsibilities. The Contractor shall be responsible for WPSs, qualification of welding personnel, the Contractor's inspection, and performing work in conformance with the requirements of this code and contract documents.
Hope this helps in defining the topic of Cert's.