The 'Other' option for the Source Node allows you to choose an individual container within Aperture, such as a Project, Folder or Album. When you choose this option a new window will pop up showing you a navigator for the current Library - click on the container you want to choose and click the OK button or press Return.

To choose a new container, select 'Other' again or double-click on the path field.

Like the 'Entire Library' option, you can choose between 'All Images' and 'Tree Structure', depending on the kind of container you selected. For instance, selecting an Album will only give you an option of 'All Images'.

Note - if you cannot see the whole of the path, move the mouse over the Source Node and the path field will expand to show the entire path.