You can exclude entire columns from the extract to minimize the size of the extract file or simply to avoid clutter with unnecessary fields. For example, you may have a description field that contains a whole bunch of text that you don't really use for analysis. You can now just exclude that column entirely.
Here's how it works
In the Data window, hide the fields that you don't want to include in the extract. You can hide a field by right-clicking the field and selecting Hide.
When you create the extract by selecting the data source on the Data menu and select Extract Data (for Tableau Desktop 6.1 and earlier select Data > Extract Data), the hidden columns are not included in the extract.
Because the hidden fields are not included in the extract, you cannot unhide the fields while using the extract. Re-selelct the data source on the Data menu and then select Use Extract (for Tableau Desktop 6.1 and earlier Data > Use Extract) to clear the selection of the extract and switch back to the original data source.
In a workbook that is using the original data source, you can unhide fields by selecting Show Hidden Fields on the Dimensions pane menu.
Note: Hidden fields are not excluded if they are being used in groups, sets, relational joins, a formula for a calculated field that is not hidden, or if there are other copies of the column from a self join that are not hidden.