I've worked for many years in the beauty retail industry and have also worked as a freelancer where I would work side by side with photographers and I will say that there are many different perspectives and aspects that can come with the term "makeup artist".
For example, many of the Cast Members that work in Sephora and even retailers like MAC go through their own trainings and certifications in order to be qualified to do makeup on the sales floor, but you also have independent folks who may be hired on by brands as their own "freelancer" or "specialty artist" where they demonstrate their artistry and learned application to further drive sales of that particular line.
Then there are also those who may be employed by a talent agency or work independently with photographers, models, or clients who may book them for specific photoshoots or special events.
There are also those who have invested in cosmotology schools to obtain their beauty license, which may also encompass specialty aspects such as skin care, nails, and hair. While I do not have a license, many friends of mine do and state that in cosmetology school, there really isn't a very strong or lengthy focus on makeup but rather the other three genres presented earlier. Businesses such as spas or salons will often time require the person they hire to do makeup to have a license just to stick with business standings and comply with local regulations based upon their business type (for example, the salon in the back of Ulta will have their esthetician perform all makeovers booked in that area as they're licensed to perform facial/skin services just as their hair stylists are licensed to perform hair services versus booking a makeover appointment on the primary sales floor with an employee of Ulta who works in the prestige makeup area). Depending on the state and even city you live in, the legalities of needing or not needing a license will vary and will outline the specifics over what is and isn't allowed to be performed due to liability issues.
There are also those who have attended and completed makeup courses or seminars hosted by specialty beauty schools or individuals that may specialize in a particular field in beauty (for example, WolfeFX may host a body painting master class for body paint enthusiasts or those looking to add that skill set to their artistry arsenal). These classes often come with a "Congrats, you've completed this course and are now certified from this brand" certificate or confirmation; however, these may or may not hold any prevalence in how they are applied. Take for example how I mentioned spas will hire someone with a cosmetology license, but if you were to walk in and say "Well, I'm certified by X company to do makeup", that won't really give you all the qualifications to get the job as you may be lacking the actual beauty license issued by the state board. Think of this particular segment of schools/learning as a feather in the cap for an artist, it's not required, but investing in improving and expanding one's skill set certainly can make for a more well-rounded artist.
With the growing use of social media today, there are also those who showcase their own artistry and talents using makeup strictly on themselves without necessarily even working with others or clients, but will brand themselves as "makeup artists" as they consider themselves artists who use the medium of makeup, but aren't "for hire".
I'm not sure if you may be looking for input from those of your specific definition of "professional makeup artist", but if you are looking for responses from a more detailed category of those who may be working with makeup or in the industry, please specify so those folks in particular will know you're directing questions to them.
I'm not sure if you may be looking for input from those of your specific definition of "professional makeup artist", but if you are looking for responses from a more detailed category of those who may be working with makeup or in the industry, please specify so those folks in particular will know you're directing questions to them.