Reading the book 'An Introduction To Continuous Optimization', I ran across the $\subseteq$ notation, but with the little bar crossed over with a small $45^o$ dash - only the bar, not the whole symbol. I don't know how to do it in latex but I hope you get the idea.
What does this mean?
Do you mean $\subsetneq$?
It's used by some to denote a proper subset and is used instead of $\subset$.