On Excluded units page you set units that won't be parsed. Typically, you put here system units, which you don't want to instrument anyway.
Units
Simple Memo field with unit names. One per line, alphabetical order is not important. Case insensitive. If you would like to add another unit, just type it in. Likewise, if you would like to remove a unit, just delete it from this field.
Clear
Clear the Units field. No units will be excluded.
Add from folder
Click this button to display folder selection dialog. All units from the selected folder will be added to the Units field.
Defaults
Resets all settings to default values (as set in Preferences dialog).