Boolean masks are often used to filter out some data based on a criterion. Boolean means true / false; yes / no; on / off. One of the easiest ways to achieve this is by using a code block. Feed the data into the code block to see if it is conforming to the criterion (or not) and then use the true / false list that results to filter out the elements as per the true or false list. Elements that conform are put into one list, while those that do not conform are put into another. Both can easily be obtained from the Dynamo node. I can think of few scripts where I have not used the Boolean mask, and I am sure you will find them just as useful.
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