A datalog file is a file that stores data as a sequence of records of a single, arbitrary data type that you specify when you create the file. Although all the records in a datalog file must be a single type, that type can be complex. For instance, you can specify that each record is a cluster that contains a string, a number, and an array.
You could use a datalog file refnum if, for instance, you were creating a subVI which will be accepting a datalog file as an input. You could use this refnum to write to the file and perform other actions on it.
J.R. Allen
