Certification is an incorrect term for the most part John.
what it amounts to is that where ever a welder goes to seek employment he she must qualify to that employers procedures.
There are different codes for different areas of work and different countries.
If you are qualified to work for a pipe line company most likely you have tested to API code but it blossoms from there in to PROCESS , POSITION, FILLER MATERIAL, BASE MATERIAL, WITH or WITHOUT BACKING etc.
Each code adresses this issues in there own way. They are generaly called "Essential Variables" and if there are changes in any one of them you must requailfy according to each code, Thus the the term "Code Welder"
So if you leave one job and go else where like to a ship yard you will have to qualify on the site, to there spec which maybe ABS, AWS or ASME.
AWS has prequalified procedures you can go to a testing Lab and weld a coupon and have it tested and you will have papers that may convince a potential employer to let you test for them. But most employers like to keep there on records,and there Procedures are there private property