sort key

     Distinctness can be enforced by using the original position of the elements in the input sequence as secondary sort key.
     Sort the elements of V, using the first two characters of each suffix as the sort key.
     Sort the encoding space in ascending order, using EC as a sort key.
     The field or fields on which the records are sorted is called the sort key.
     The sort key is a variable or expression to be used for the sorting, and the modifier specifies a sort direction, ascending or descending.