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.

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This Source Node option allows you to work on every image in the Library.

All Images will simply pass every Version in the Library to the rest of the workflow.

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This setting will run the workflow on all images currently selected in Aperture.

Note - if you have multiple Browser panes open at once, only the selection in the active pane will be used.