A swap disk creates extra "ram" by borrowing hard drive space to "swap" out part of what is stored in ram. A ram disk creates extra hard drive by borrowing ram space to present as storage. The two provide different functions which are closer to a mirror of each other.
Since a swap disk is a blob of file on a hard drive and a ramdisk is a simulated hard drive using part of the system ram, you can host your swap file on your ram drive. I'm not sure what benefit there would be over simply using the full amount of ram as physical ram rather than swap ram on a ram hosted virtual drive.
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