I'm currently sorting through my CC and started off by taking it all out and moving it into a separate folder. Naturally, all my savedsims (.sim) now show up with charcoal skin, basic brown hair and EA clothes.
There are three sims however which I downloaded. They're still sim. files, but they're not mine that I've saved in game. These three packs seem to have their content attached to them, and therefore they look completely normal in the game and I also have their hair, skin, clothes and accessories in game too.

I was just wondering how these sim. files are different to the others I've saved myself. I would love to find a way to save my sims along with their content as I forever fear losing it (as I have done in the past.) But for now, I kind of want this content out of the game, without actually removing the sims themselves.

My question really is what is the difference and how do the ones I downloaded have content attached to them, and the once I've created myself dont?

This is somehow related: http://www.modthesims.info/showthread.php?t=583677

As to how it is done, I suspect that the creators of these Sims used perhaps something like S3PE to merge the Sim and the CC into one *.package file and renamed it to *.sim afterwards. At least I can't think of any standard process to achieve this.

Actually, I think you can get your sims with the CC attached, if you save them in CAS with the Share option and export them as a sims3pack first (example: testsim.sims3pack).
Then just use Delphy's multi-extracter to extract the sim from the sims3pack. You'll obtain a package file that you'll have to rename by changing ".package" to ".sim" (example: testsim.package will become testsim.sim).
Once you've placed your sim in the SavedSims folder, even if you don't have any CC installed in your Mods folder, your sim should still be fully equipped.

In any case, it did seem to work just now, when I did the test. Little issues, I had to copy the makeup back to all outfits (easily done with Nraas Dresser) and the first time I changed my sim to Outwear, the game created a second outfit, instead of changing into the already existing one. But it worked fine the second time.

I just want to add that all my CC was installed as merged packages.
But I don't know how fullproof this is as I don't use this method at all, and I don't know how the game would react if two sims (or more) had the same CC attached to them.

Thanks so much both!

Blyss I had tried what you said, exporting the sims and extracting with multi extractor. But I kept it a package and again the cc wasn't attached. I'll give this a go now

Don Babilon thanks for the link, I'll read that one now too!

Oh my, this is one of those duh moments. I haven't used that share button in CAS for so long that I completely forgot about it and have been wrecking my brain over how this could work in the most twisted ways while all the time the solution was so simple. So, yes, it worked, and in my case the only things that did not export were the default replacement skins and eyes, which is kind of logical, I guess.
However I doubt whether this is working as intended, as, first of all, it allows sharing of Store stuff, and, secondly, I consider a Sim with attached CC as being a potential risk. In my case CUSTARD didn't even report the Sim as corrupt as was the case in the thread I linked to above.

Edit 1: It doesn't seem though that it works every time. I do have some Sims downloaded as sims3packs, converted them to .sim and none of their cc has traveled with them. Or at least not fully. S3PE shows that one of them has a hair attached, but it doesn't show in CAS. Besides what conditions must be met for the CC to allow it to attach itself during export? If every package travelled inside a Sim whenever you hit the export button we should have this as a more widespread phenomenon but the other Sim I tried that should be full of CC had none of it, neither as separate package (which means that it was installed as sims3pack), nor attached (installed as package).

Thanks for looking into this more thoroughly, @Don Babilon!
I really am not sure what are the determining conditions in making this process successful.

Even if I rarely download sims, I always convert sims3packs and now that I think about it, what I've encountered when it comes to sims looks a bit inconsistent, just like you've observed yourself. Sometimes clothes come attached, but not the eyebrows. Sometimes everything is there or nothing at all.

Following a Sims 2 logic, "packing" your sim should actually serve the purpose of getting the sim and all its CC into one file. But I have to agree with you, when you say that it doesn't seem to be what they intended to do with Sims 3...

I really don't know what factors are at play here. Maybe packages that get extracted separately from the .sim were actually installed via the launcher and retrieved from the DCBackup folder, a bit like what happens with lots? For the rest, I really don't know. Could it be a question of patch level? An unexpected side effect of a mod? A bug EA never looked into?