As I understand it, 'Files Forever' is a service, separate from your hosting, that allows you to store files for a fee (currently a once only fee of $2 per GByte) and allow others to download them. You can, if you choose,set a cost for the file, which others will have to pay before downloading them and DreamHost will handle the billing for you.
I can see this as a great way to distribute shareware applications etc.
Since the file is transfered from your hosting space to a 'Permanent online file archive', I don't think any downloads of that file will affect your bandwidth quota.
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