True archiving keeps all the various versions of a file accessible at all times so that you can always go back and retrieve the file you want in the state that it was in on Wednesday, even if you've archived twice more since then. The main differences between Backup and Archive then, are those that make it easier to deal with more complex operations and multiple files. Selecting a Source and an Archive is exactly the same process. Selecting files isn't any different, but you are more likely to want to individually select which files go into an Archive than you are for a Backup.
The easy way to select which files will go into the archive is manually. If you click once on a file, it hilites, but this does not mean it will be archived. To go into the Archive, it must be marked with a check, which occurs when you double-click (which also removes checks on pre-checked files) or when you choose Hilite Marks from the Browser menu. Retrospect supports the standard Mac methods of selecting multiple contiguous files with a shift-click and multiple non-contiguous files with a command-click. In addition, double-clicking on a folder either hilites or unhilites all the files in that folder, so manual selection is painless. It's a bit harder to set up a Selector to select files automatically, but once you do it, it's always there. Retrospect ships with a number of standard Selectors that cover many situations, and you can create your own custom Selectors to take care of any other situations you may find. The standard Selectors include All Files, Graphic Files, Modified in Last Week, No Change in 2 months, No Change in 6 Months, No Change in One Year, No Files, Only Applications, Only Documents, and Only System Folder. I'll cover the sort of things you can do with custom Selectors later on.
Once you have selected the files that you want archived, you come to the Options screen again. It looks mostly the same as the Options screen for Backup, but also has a check box for Move files (which means that after it archives the files it will delete them). In addition, you can switch the screen to the Extended Options, which includes even more useful options. The first two, Scan To Compare Source To Archive and Don't Add Duplicates To Archive are checked by default. The next one, Only Match Files In Same Folder, is useful if you move a file after it has been added to an archive. If this option is checked, Retrospect will consider it a new file and will archive it again, even if it hasn't been otherwise modified. The next checkbox, Store Snapshot, allows you to Restore an entire hard disk even if you created the archive with Archive instead of Backup. I generally use this because it would be a pain to have to manually select the latest versions of everything and place them all in the correct folders after the fact. You also end up with files that were archived and then erased if you don't use the Snapshot. The next bunch of options are less useful, and I'll admit to never having used them much. You can have Retrospect set the backup time for volumes, folders, and files, which can be useful for working with Selectors, but since other Mac applications sometimes change that backup time, it isn't necessarily reliable. You can have Retrospect report open files rather than archiving them, store archived files separately from already existing files in the Archive, and finally, when moving files, you can have Retrospect delete empty folders, a nice touch. The final item in the Extended Options is an important one. It is a pop-up menu with Selectors, both Custom and Standard, that affect what Retrospect will compress. I'd always ignored this until I started using DiskDoubler heavily. It took Retrospect a long time to deal with compressed files, and if anything, they might have become slightly larger when it tried to compress them again. So I created a Selector which tells Retrospect not to try to compress any files created by DiskDoubler, Compact Pro, or StuffIt Deluxe. That saves time and space, both of which I'm lacking in sufficient quantity.
OK, so you've figured out how to Archive files for later Retrieval. Let's see how you would go about getting a file back if you wanted to see an earlier version or if your two-year-old accidently erased it. First choose Retrieve from the main window, then pick the right Archive, and the right Destination. Then you come to a file Browser that's almost like all the others you've seen. The only difference is that you have to pick which session to get the files from as well, since that's how they are organized in your archive. Once you select a session (which is just an entry in a scrolling list to the left of the file list), you can mark the files in that session that you want to retrieve. Of course, if you want, you can create a Selector to pick the files for you, although I'll warn you that you have to think carefully about the logic involved in a Selector to retrieve files. You also have to check the Search Parameters to make sure that Retrospect is using your Custom Selector and that it is looking at the correct session or sessions. It took me a while to get the hang of it, but it's not difficult once you've done it once or twice. After you've selected the proper files, you get to the Retrieval Options. These options primarily affect how files will be stored on the Destination volume and whether or not you want to replace existing files. You can also customize the retrieval and use a finer filter on what you want to happen. I won't go into the gory details, but suffice it to say that you can achieve just about any effect you desire in terms of how and where the files go.