"Certification" in welding is a little bit different than you're probably thinking. It's not like being certified as a nurse2 or as a pilot. It's not a professional license.
There are all kinds of certifications. A certificate only says you're able weld one specific metal with one specific process under a specific set of conditions. They don't transfer from employer to employer. Most companies will have you take a new certification test before they hire you. If you get a new job, you take a new cert' test or tests. If you start work on a new material or with a new welding process even at the same job they'll usually want you to take a new certification test with that material.
You can't certify in "combination welding." You can certify in, for example, Flux-core welding in the vertical position, with flat structural steel plate less than 1" with a groove weld. Or, you could certify in TIG welding stainless pipe in the diagonal position, greater than 1" thickness. Or you could certify in MIG welding aluminum plate less than 1", in the horizontal position with a fillet weld. You get the picture.