botanical-garden and z*garden emphasizes all nodes from the target that rate high for both botanical garden and zoological-garden;
botanical-garden or zoological-garden emphasizes all nodes from the target that rate high for either botanical-garden or zoological-garden;
not botanical-garden emphasizes all nodes from the target that rate low for the botanical-garden (and perhaps high for the things you are looking for -- try queries like not zoo* and not plants, then not zoo* and not plants and not fungi to see this effect).It is possible to normalize ratings using e.g. norm botanical-garden. Normalizing means to put all terms on a par -- for example, the top result for botanical-garden has a rating of +0.88235 (fairly good on a scale from minus one to plus one) but that the top result for zoological-garden has only a score of +0.29412. Notwithstanding shortcomings in the foregoing evaluation process, this expresses the fact that not each single term will have a most likely target that scores globally among the top ratings related to all terms. In order to make up for the relative differences, say e.g. norm botanical-garden and norm zoological-garden to find targets that are important for both terms assuming an equal standing.