Each site's FAQ makes it very clear what the site is about and what kind of questions are expected.
For example, Stack Overflow's FAQ says
Stack Overflow is for professional and enthusiast programmers, people
who write code because they love it. We feel the best Stack Overflow
questions have a bit of source code in them, but if your question
generally covers …
a specific programming problem a software algorithm software tools
commonly used by programmers practical, answerable problems that are
unique to the programming profession … then you’re in the right place
to ask your question!
while something like IT Security's FAQ says:
IT Security - Stack Exchange is for Information Security professionals
to discuss protecting assets from threats and vulnerabilities. Topics
include, but are not limited to:
web app hardening network security social engineering, including
phishing risk management policies penetration testing security tools
using cryptography incident response Questions on setting up your home
PC antivirus may be more appropriate over at superuser.com; and
questions on the deeper aspects of cryptography belong on crypto.SE.