Sometimes the action shall not be performed on all sets. The numbers of sets can be specified in a list. +l is leading the list of sets if you specify the sets on which the action shall be performed. The list has to end with a -1 and each number has to be separated with a blanc. The counting of the sets starts with 0.
This is the only time so far, an option does not start with the - character but with a + symbol. The reason for this exception is that the -l option has the opposite meaning. Somtimes it is more convenient to specify only the sets, that one does not want to be changed by the action. In this case a list of sets can be specified between -l and -1.
If no list is given, all sets will be taken for the action out.