B5.1 is the qualification document the defines the requirements that have to met by a candidate seeking certification by his or her employer. The contractor (employer) would develop a certification program where qualifications per the requirements of B5.1 must be met.
The candidate is required to pass the requisite examinations, developed by the contractor, to demonstrate proficiency. If the candidate meets the qualification requirements, he is certified by the contractor. It is very similar to qualifying and certifying welders in-house using ASME Section IX is the governing document.
The examinations would include questions from each subject group, a hands-on practical examination would require the candidate to evaluate (correctly) several weld specimens that contain various defects and discontinuities and the candidate would be tested on his or her ability to navigate a code (the one used by the contractor in day to day operations would be reasonable) by answering specific question based on the code.
As noted, it is much like the methodology described by SNT-TC-1A and similar to the CWI, but it is entirely an employer based certification that would not be transferrable to a new employer.
Best regards - Al