Evenly Split Columns
When you check this box, the table in the HTML file will be split into even columns based on a percentage of the page width. For example, if you choose 4 columns for your file each column would be assigned 25%. If you chose 3 columns, then each would be assigned 33%.

If the box is unchecked, then the browser will automatically size each column based on the amount of text in each column. So you might end up with one really wide column, one very thin column and one average column if you had 3 columns in the file.