Everyone is some range of introverted to extroverted, for example. It's their preference, such as the flavor of ice cream they like. It can change often, under circumstances, or even not be present, of course. It's not set in stone; it's just a preference. It doesn't define you; you define it.
Myers-Briggs is unfortunate in that it assigns that preference as letter. You like Chocolate. You're a C. There are some who have no problem with this sort of designation. They love chocolate always and in every circumstance. Many, however, are not one letter, and so it's a poor test.
The point is not the letter. The point is asking yourself the question: do you feel more energized after a dinner party or after a quiet day alone? The point is to know, not just what flavors your prefer usually, but what situations (in this case) you prefer. And why not know that about yourself?