When linking objects to files, ToDos or URLs, the corresponding icon is then shown on top of these objects in the 3D view.
Links are also visible from the attachment context menu, where related ToDos and files are also shown. ToDos and URLs can be removed by clicking the X button on the right of the attachment.
Links icons in 3D can be shown/hidden by checking/unchecking "Attachments" in the visibility control dropdwon menu located in the toolbar.
If the attachment visibility is off, the attachment icon will still display if an object is selected that has an attachment.
Next Topic: Using the Attachments Panel