Sorry to be slow to answer this, but I hope the answer is interesting. Myers Briggs practitioners work in all sorts of areas of society. I'm a church minister - I have used it in churches, with disruptive teenagers in schools, in the armed forces, and in individual and couple counselling.
'Qualified' status is earned through what is described as a 'graduate level training programme' - this lasts about 160 hours, and includes preliminary reading, an intensive residential programme, completion of an exam, and a period of evaluated practice ending with a final residential day. The training is tough: not everyone passes, and in fact two people in our training group had breakdowns during the training period. Study covers the theory and correct application of the instrument, statistics, ethical uses of MBTI, and the use of the MBTI in situations such as conflict management and team building.
Practitioners are registered, work to a code of ethics, and are able to buy and use restricted test materials. Control of the materials is necessary to ensure consistent quality of the experience, and because wrongly applied the theory can be at best confusing, and at worst can seriously mess up people's heads. Trust me - I've seen it happen!
I hope that helps to explain what an MBTI practitioner is and does. I'm always happy to answer questions about what we do - just ask!