A file data source is very similar in spirit to a spreadsheet with imported data from an external file. The difference is that the imported data cannot be shown and edited in LabPlot after the import anymore. This can be sufficient e.g. if you only want to plot the data stemming from a calculation in an external program (and exported to an ASCII-file afterwards).
Since no spreadsheet has to be filled with the imported data, the import into a file data source is faster than into a spreadsheet which can be advantageously when dealing with big files.
It is possible to store the link to the external file in the project file only and not its content. Each time the project file is opened in LabPlot, the content is read from the external file again. Also, it is possible to let LabPlot watch the file for changes - the content of the file data source is updated if the external file was changed.
The additional options determining the import of the data are equivalent to those provided in Import Dialog.