Sign of completion from a piercing mini-course.
Some piercers (and a smaller number of tattoo artists) will hang diplomas stating that they have graduated from a piercing course such as the ones offered by Fakir Musafar or Gotham (formerly Gauntlet).
It is important that you understand exactly what these diplomas mean. Best case, they simply mean that the person attended the piercing course mentioned. It does not mean that they are qualified to pierce. These "piercing schools" will tell you that they are meant as pre-apprenticeship preparation courses.
It should also be noted that these piercing diplomas can be ordered from magazine ads, and have no meaning whatsoever in that case.
A certificate that you may see in a very small amount of studios are old Gauntlet "Master Piercer" and similar certificates. These are quite meaningful because they indicate not the completion of a weekend of training, but multiple years of piercing at Gauntlet.