When you start a torrent, you "announce" yourself to the tracker to get a list of peers and also to be available for other peers. While downloading, you will periodically report to the tracker to update your current status and to get more peers if needed.
Manual announce simply forces the client to update your status on the tracker (with some timeout to reduce tracker load). You can use this function, e.g. when you get disconnected from your ISP and you want to update your new IP on the tracker.
With external announce you can connect to another tracker that serves the same torrent you are downloading or you can use it when the orignal tracker url (or port) has changed and you don't want to redownload or modify the torrent file.
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