Raster images can be placed by dragging and dropping them from a web browser, either as an embedded or a remote resource.
When an image is placed as a remote resource, its source address (URL) is recorded in your document. Affinity detects when a remote resource has been modified and will either notify you or automatically update the resource, according to the auto-update behaviour setting in the app's General preferences.
A document's remote resources can be embedded at any time, e.g. when you are ready to send it to a professional printing service or want to create an archive of a publication. The document will retain its record of a resource's source address, which enables it to be made remote again if needed.
Any image from the Web that is initially placed as an embedded resource does not have its source address recorded and so requires you to perform the same steps again if it needs to be updated.