Converts as many spaces at the start of each line into tab characters as possible, without changing the layout of the file. Spaces that occur after a non-whitespace character on a line are not converted. This function will only have its desired effect when using a fixed-width font like Courier New. If you select multiple lines of text first, only the the selected block is converted. If there is no selection or the selection does not span multiple lines, then this conversion affects the entire file.
The number of spaces used for each tab by the conversion is the same the number of spaces used for each tab to display the file. It can be set in File Types|Editor.
Convert|Tabs -> Spaces