According to Stack project, when referring to them I should use the Tag system
<https://stacks.math.columbia.edu/tags> since



> 
> “The tag system provides stable references to definitions, lemmas, propositions, theorems”
> 
> 
> 


and



> 
> “The place of the lemma in the document may change, the lemma may be moved to a different chapter, but its tag always keeps pointing to it.”
> 
> 
> 


Is it really true though? The stack project has many corrections every day so what happens if I refer to a Lemma whose hypothesis or conclusion suddenly change? Can we still consider this as a “reference”?



Can/should I cite some Stack projects Tag in a paper
Yes, you should cite Stack Project Tags! The conclusion of the Theorem/Lemma/etc shouldn't weaken over time.


Compare to that old reliable citation "personal communication", or, worse, to a paper that is "to appear", and is never written. At the very least, the proof is there to be examined, and indeed the entire proof tree back to elementary material.


