Constructed languages are opposed to natural languages, which means they didn't evolve through time but rather were created by a single person or a group for some purpose. (Note: Read Tag Wiki about this Tag usage)
What questions should have this tag? If your question is about constructed languages only or mainly, then consider posting on the appropriate proposal about constructed languages. In Linguistics SE, questions about such languages are off topic, and you might risk having your question closed. If you're in doubt, ask on Meta or ask some users on our chat.