DataItemIndexList Property

For each element this can specify an index into another list where the data "belongs" to.

This can be null (if no reasonable correspondance exists or if the correspondance is trivial and therefor not needed ([0,1,2,3,...])).

For data on a reach, the element at index i in the IndexList having IndexList[i] = j is used to specify that the value belongs to GridPoint number j in a list of GridPoint's.