The current logic is AND between Focus Tools and OR within a particular Focus Tool. So if I want Blues music between 1970 and 1975 Focus will show me albums which are Blues AND 1970-1975. Conversely if I Focus on both Blues and Classical, I will get Albums which are Blues OR Classical.
Remember also, that clicking on the + sign of a Focus toggles it to - in which case it will show all Albums except those satisfying the Focus.
User tags are treated as a Focus tool, so if you want to use AND find a Focus criteria which best suits your needs rather than adding a further User tag to the Focus.
There is a Feature Request to enable more user defined logic to Focus searches.