If you study biology, you study life. And when asked about what you study, you will want to be more specific, so you might say you study a particular domain. If the question persists, you might even go into kingdoms, phyla, classes, orders, family, genera or even species - in that order.
When studying life, these things matter.
If you happen to be human, you study humans. And when asked about what you study, you will want to be more specific, so you might say you study a particular domain. - And so forth, in that order.
The first order of differentiation you do is this: do I want to have sex with this person or not? And depending on whether or not you want to, the rest of your interactions with this particular human are colored by it.
The second order of differentiation is if it's a boy or a girl.
After that, you add class, race, appearance, personality and all those other things that makes up the impression you have of the human in question. In more or less that order.
Humans are strange creatures, are they not?