What is the meaning of $\sqsubset$ and $\sqsubseteq$? Does it have any special meaning, or is it just an alternative to writing $\subset$ and $\subseteq$ respectively (for proper subsets and subsets)?
I have been looking for an explanation everywhere, but so far I could not find it. This may have to do with the fact that I am not even sure what this symbol is called (makes it difficult to search for it), but I have tried several things (like searching for $\sqsubset$ on this site), and nothing came up, other than lists of mathematical symbols for LaTeX without any explanation.
I have seen it used in papers (e.g., http://www.cril.univ-artois.fr/~marquis/everaere-konieczny-marquis-ecai10.pdf on page 4, footnote 5), but never explained. I am starting to think that $\sqsubset$ and $\sqsubseteq$ are equivalent to $\subset$ and $\subseteq$. However, sometimes there are subtle differences, so I want to be certain about this. I want to be sure that I understand the intended meaning when reading future papers, to avoid any misunderstandings.
Thanks in advance.