The script reads a list of genomic coordinates and output all the features with specific tag values. If you want to know more about tags, please consult the GFF format page: http://www.sequenceontology.org/gff3.shtml
The tools reads the input file, and more specifically the tag that you specified. You can mention a lower and a upper bound for its value, or a specific value, and the tool will print all the features such that the tags are between the specified bounds or matches the string.
A tag has to be present for each feature. If not, you can specify a default value which will be used if the tag is absent.
This tool can be used to select the clusters with a minimum number of elements (the tag nbElements counts the number of elements per clusters) or to select the reads which have mapped less than n times (the tag nbOccurrences counts the number of mappings per read).