It will create a SWAP file on a partition of your choosing and that will be your additional "virtual RAM", which certainly helps when RAM is scarce.
Remember, I mounted /media/cf for precisely that purpose. Namely, flash is too small and HDD has moving parts, so SWAP file can not/ought not go there.
See the images attached...
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"Rarely do we find men who willingly engage in hard, solid thinking.Their is an almost universal quest for easy answers and half baked solutions. Nothing pains some people more than having to think." - Martin L. King