The Logic OP Node can accept a front/back/matte input. You can perform Logical Math blending operations. Add, Subtract, Multiply... and much more complex math. But basically blend modes and simple composite of A over B with a matte if needed.
I tend to commonly use it to alter matte clips so that I can combine various mattes together and create new mattes through different blendings.
Senior Editor - Autodesk Smoke
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Brian's blog is a great resource. Very concise, and full of information on how you'd actually use the various nodes in production.
There is also the Smoke manual, that can be viewed online or downloaded as a pdf. This has stuff that Brian hasn't yet gotten around to covering :) and is also very in-depth and good. Just ignore the occasional strange looking blue-grey interface screengrabs!
I use Logic Ops all the time, it's one of my goto nodes when we're in a creative session.
Joel Osis has a video showing how he uses them during color grading: