Sale by Set Date is just a copyrighted term used only by Barry Plant.
It is simply Private Sale but they've changed the name, as lil skater suggests, to create urgency so the vendor has finite time frame rather than the usual wait for an acceptable offer to come in.
Don't be fooled though, just because it says sale by set date doesn't actually mean it has to sell by then. I've made offers on properties with this method which have gone over the deadline as the agent tries to drum up other offers.
The key here is to always set a tight deadline on your own offers and threaten to walk if your terms aren't taken seriously.