This letter is nothing new to Mark because he has already received 10 different letter like it. Some of these letters were in response from some of his applications, others were standardized and only unique considering it had been specifically addressed to Mark. In short, it was a letter for a job offer at a hospital after Mark graduates college. What? Did you expect a random nobody like Mark to receive a fluke letter inviting him into the UNHA? Even the lowest janitor at the UNHA is at least required to be a B rank and considered a reserve member of the heroes there. It was a much more mundane letter in comparison, only noting on Mark's "specialties" which he constantly showed off to both impress his professors and recruiters in order to have a job after college. Like many other people of the F rank, Mark's parents work two jobs and have never received a college education because without perfect grades the only other way to get into any college is through one of the Big Three. Money, Power or Influence.
Except for Mark being an exception because of how hard he has worked to get there with impeccable grades, most other college students were only able to get in through paying an enormous fee, have a ability ranked high enough where their grades were less of a concern, or if their family had connections. These connections could either be from a family member being an alumni, a family member with at least a A+ rank card, or a family member with UNHA status which means everyone bends over backwards to get on their good side. Mark unluckily had none of these things being the first one from his immediate family to even have a chance at going to college instead of immediately entering the workforce.
Mark's specialties lie in how he has both researched his power extensively and marketed himself according to that research. All of these tactics with almost perfect grades let to him being able to barely slip his name into many job requirements albeit none of them as prestigious as the most recent letter. Mark has marketed himself as a doctor since he first understood the application of his powers. "Why a doctor?" Some of his fellow students may have asked led to a further discussion of Mark's abilities or lack thereof. As a F class rank Mark has no combat power as compared to even a normal trained human. For the longest time the only thing Mark was able to do was use his infrared radiation to start fire in any of a number of adventure games he played in VR or heat up some of his food if it got cold. As a 10 year old kid with dreams of becoming similar to one of the 4 masters this was totally unacceptable, and led to young Mark experimenting with ideas well beyond his 10 year old mindset. This eventually paid off with the help of extensive research by both himself and his kind grandfather led to the discovery of his current abilities which were perfect for the medical field.
X Ray Scanning
Using a combination of Mark's ability to control ionizing radiation and see in almost all visible light spectrums along with his own radiation, let's him act as both a X ray generator and detector.
This power makes him similar to what many people call a X ray machine. The standard X ray machines consist of two parts. One part produces the X rays and sends it forward through a series of vacuum tubes to the patient. The other part of the machine is a X ray detector which involves see the radiation when it is absorbed by the bones or teeth in order to have a clearer picture. As many doctors would prize the ability to have someone instantly be able to X ray patients and then "pull" out the residual radiation after they are finished, this ability after being showed by Mark would definitely warrant a second glance.
Mark's second "ability" he marketed was less valuable in this day and age but still relevant. Chemotherapy.
Chemotherapy
Mark is able to produce and control his radiation to saturate an area of the human body in order to destroy foreign organisms or tumors present from diseases or conditions with cancerous effects, then reabsorb the radiation but no long lasting side effects to the patient.
With the many discoveries of Igor Karma before the war, and subsequent research by many brilliant minds after all these years, Cancer was a thing of the past with only the poorest of residents or people outside the cities even having the chance to develop it in their lifetime. The realest problem normal city residents and even the police and guards of the cities have to face which impacts their health are parasites. With Mark's control over Gamma radiation he is able to irradiate small organic matter that is left in the body after using his X ray ability. This is perfect for any sort of fungus or organic disease and even leeches. Why are leeches, parasites and fungus so important to be protected from in 2479? Even the poorest and most destitute of the citizens of a mega city never goes outside because of these problems and even to think to do so is considered a form of suicide. This is because of the single largest problem humans face after the effects and byproducts of WWIII.
Hyper-aggressive mutant beasts who live and thrive in the irradiated areas outside the mega cities.