The Holland Codes are a way of describing work interests using six types: Realistic, Investigative, Artistic, Social, Enterprising, and Conventional. Take the first letter of each and you get RIASEC, which is why the model is known by both names.
John Holland, an American psychologist, proposed the framework in the 1950s and refined it over the following four decades. His central claim was simple and has proved durable: people look for environments that let them use their skills and express their values, and satisfaction comes from the fit between the person and the environment rather than from either one alone. Jobs, in other words, have interest profiles too, and the useful question is how well the two profiles overlap.
Holland arranged the six types on a hexagon in the order R, I, A, S, E, C. Types next to each other on that hexagon are similar, and types opposite each other are the least alike. That is why a code like RIA feels coherent while a code mixing opposites, such as R and S, describes someone with genuinely divided interests who usually does best in a hybrid role.
The model matters far beyond academia. The US Department of Labor publishes a RIASEC interest profile for every occupation in the O*NET database, which is what makes it possible for a test like this one to hand you real occupations rather than adjectives.