A non-random selection is one in which all of the people do not have equal chance of being included in the sample.
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It is when you specifically choose the people that you "sample", in order to collect data about them. (As opposed to picking numbers representing people out of a hat, etc... ). For example, if you wanted to collect some sort of data from every male currently living within the same postcode as yourself, that would be nonrandom. "Non-random" varies a lot depending on what you're actually sampling. What would be "random" in one context can be "nonrandom" in another.
selective
Combination
There are many selection tools in Photoshop including Color Range. For Tools take a look at Options Bar and move arround icons until you see pop-up tooltip Subtract from selection, then press that icon.
Natural Selection!