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. Sharia also encompasses daily transactions, customs, habits, fiqh, and 'ibadat, in addition to law.  
-> 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 extractions of 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 (or Islam) 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!