You need a defensible way to define what your test will even cover. This starts with a job analysis. A committee of subject matter experts (SMEs), will generate a list of tasks in the profession, then we obtain quantitative data from incumbents on which tasks are most frequent and important. This data is invaluable.
Certification accreditation seems like two of the same word! But certification refers to the process of attesting to the skill of a professional, like taking an exam to be certified as an accountant. Accreditation is the process of attesting that a program or entire organization meets certain quality standards. So here, we are putting a stamp of approval on a certification that it is accredited to be high quality.