Each data source managed by a data source provider is specified by means of some set of data source specific parameters. The parameters proc of each data source returns a Tcl dict that describes the set of parameters expected.
The keys of the dict are short parameter names (used by the data source to look up the parameter in the dict). The value of each dict element is is a human-readable description of the parameter. Normally a graphical user interface would use this string to prompt the user for a value.