Is that simple enough for you? Or, more accurately I might ask.. is that TO simple for you? ;)
It's not mere arbitrary label. It is an ethical category -- who do you owe your labor to. It doesn't matter ethically if it is pre-tax or post-tax -- if society determines who you owe your labor to, that is socialism. If you are free to exchange your labor (and therefore keep its fruits) as you see fit, it is individualistic capitalism.