It is recommended that people ages 12 and older, who are moderately to severely immunocompromised, complete a 3 doses series of an mRNA vaccine . This additional dose (the 3rd dose) is necessary because their immune systems are compromised and may not mount adequate protection from 2 doses.  The additional dose is considered to be part of their primary series.
A “booster dose” is a supplemental vaccine dose given to people when the immune response to a primary vaccine series was adequate but is likely to have decreased over time. it is not unexpected that even those who have healthy immune systems would benefit from a booster dose (just like we all need Tetanus boosters, Meningitis booster, Chicken Pox booster (Shingles shot) and a Pneumonia booster (at 65yo))