@Kiro Its just like there is
Mathematics and Mathoverflow. One is for general questions, ranging from elementary to academia, the other is for questions
from researchers in the field
to researchers in the field
about research level stuff. To be clear
Computer Science is the general site, while Theoretical CS is the research-level site with a slightly more narrow scope to be only about Theory (this I think is to be both Algorithms/Complexity and Formal Language theoriy and related fields)