This is my understanding, please correct me if I'm wrong or comment if you think I should go in details or add some details:

 - Shari'a is the law of Allah which basically is compiled in the copies of the qur'an. One could add established or approved (authentic) sunnah to that.  
-> shari'a is more general and shouldn't be tagged solely -at least not that often-!
 - Fiqh is the application of the law or more exactly the science on the interpretation of the texts and and extractions of a verdicts based on the sources of the law or jurisprudence school (osol al-fiqh). Here one may rely on more than the qur'an and sunnah.  
Before declaring this as a synonym of [tag:halal-haram] we must understand that fiqh often is more than that: see for example the issue of inheritance ...
Also in fiqh we may present options and solutions: What to do if one missed a prayer on time, what to do if one was traveling and wants to pray ...  
-> fiqh is more specific!

So a question on a verdict is basically a question on fiqh [tag:fiqh].

A question on what the law says is a question on shari'a [tag:sharia].

The second option ([tag:sharia] alone without halal-haram or even fiqh) should therefore rarely be tagged! 

However when the question is what does Islam say about "X"  or "halal-haram"-Questions (if a general ruling in the means of what does Islam say about ...) is asked, users have the law in mind (as most don't make a distinction between shari'a and the jurisprudence and verdicts) not a special verdict!