STL synonym created instead of burnination: See stl
It appears that stl is "selectively" used for questions that utilize the Standard Template Library. However, this only applies to C++, and such code should already make use of the STL. Even if a C++ question doesn't utilize the STL for whatever reason, omitting this tag doesn't send any kind of signal. With over 2000 c++ and only 59 stl questions, it hardly adds anything to the question.