walter628 Loc: tahlequah, ok

when I work with raw files in pse it makes a file with an xmp extension. Anybody want to explain what and why? I cannot open this file so I don't understand what it is for.

KG

It's a sidecar that holds all data about what settings you applied to the raw file in post processing.

arphot Loc: Massachusetts

Much like Photomatix software for hdr; they also have an xmp file that holds settings, in this case for the tonemapping.
You should be able to call it up when you want to use the same settings on another image by importing it into the preset settings.

bobmielke Loc: Portland, OR

It's an information file that is created when you edit a RAW file. It can be erased but should you open that RAW file again you'll have to start from scratch.

walter628 Loc: tahlequah, ok

ok, you say it is a information file. info for who?? I can't open it. Is it saving the info for the next time I open that file? What I am asking is, are the changes I make to the file the info that is being saved?

charlessmall18

What is needed here is a bit of programmer's lore. If all you have done used a word processor, an MP3 player, JPGs, and an e-mail program, then, from you experience, you might assume that everything always gets stored in one file. JPGs often has EXIF data embedded in them in addition to pixels. MP3 files have all the "tags" embedded along with the music (artist, title, genre, lyrics, cover art etc). (the MS Word file format is a baroqe mess that often collapses under its own weight if you try to make a big file with a lot of pics in it and use all the formatting stuff like a TOC and an index simply because everything that has been klooged into the file format over the years -- all in a single file). But in general, programmers can choose to separate different classes of information about a single entity into different files. Databases are most often have lots of files that get opened in the course of your running a database program. What you have here is some data (raw pixels) in one file and some ancillary data in another file. When you "open" the main file, the ancillary file gets opened by the program (unbeknownst to you) and it contents factored in. There is no point in opening the ancillary file without having the main file. So you can't open it. It would seem that you program will soldier on without the ancillary file but they have been telling you not to erase it. You cannot buy a small hard disk any more so why are you erasing mystery files?