About metadata and XMP

Question

This question was posted in response to the following article: http://help.adobe.com/en_US/lightroom/using/WS638E3AC9-A04C-4445-A0D3-F7D8BA5CDE37.html

Answer 1

I would draw a very clear distinction between the concepts of "backup" and "archive".
Backup is a continuing and regular process, referring always to your most-current live and changing data, and is designed to protect the continuity of that data. This allows you to recover from a problem or accident, whether externally caused or the fault / inattention / reconsidered action / whatever of the user - and get back to where you were beforehand. That might relate to all your data together, as in a disaster of some kind, or selectively.
Archive is a planned process that defines certain data (or a copy of that) as non-current - often, as a matter of unchanging record. This gets put in a place where it can be separately accessed as needed, but often that is accessed by a different means than you access your live data (for example, you have to open a different Catalog).
LR's standard backing-up of the Catalog, also externally copying that to other physical media when LR is not running, as well as making an external copy of all your Raw files etc to other media, all deliver a backup facility - which is only useful if done regularly.
Exporting as Catalog - with negatives - (say) all your images from the year 2007 into a specially labelled and distinct disk drive that then sits on a shelf, is an example of "archiving" as I would use the term. That might be done for its own reasons, even if you also retain some or all of this inside your working data set too. For unchanging data, it doesn't particularly matter when you do this.
[edit: I should have made clearer, the images contained in an exported separate Catalog can be imported into - merged with - another Catalog if that becomes necessary.
If you have kept a backup (older) duplicate of your full working Catalog, you can go into that and get at information which may have been lost in the working version. You can save that out as a new temporary Catalog and use that to transfer things betrween Catalogs - which is effective if sometimes laborious - or for specific images, you may be able to get what you want more rapidly, by writing metadata to the files on disk, and then in the other Catalog, syncing or otherwise re-reading the metadata (or as a last resort, if the images / folders are now not present there at all, fully importing them).
But if it is just a question of "rolling back" the editing of particular images, the History option lets you do that right now, using the Catalog you already have.]