A plain edge is a continuous unserrated cutting edge. It cuts cleanly, is easy to sharpen with ordinary equipment, and is the right default for almost everyone who is not cutting rope for a living.
Push cuts, draw cuts, detail work, food, and anything where the quality of the cut surface matters. Because the edge is one continuous line, it separates material rather than tearing it.
Sharpening is the other big advantage. Any flat stone, rod or guided system will maintain a plain edge, and the whole edge can be renewed in a couple of minutes.
Thick rope, webbing and heavy fibrous material can defeat a plain edge once it is anything less than very sharp, because the fibers roll away from a smooth edge instead of being trapped.
If that is a regular part of your work, a serrated or combo edge earns its keep. Otherwise plain is the better tool.
The term is not academic here. 33 of the products currently on our shelves describe this feature in their own manufacturer specifications, across 5 of the nine brands we stock. Examples in stock right now include Hogue Large Tactical Drop Point, Hogue Ex-F03 Fixed Blade Hawkbill, Gerber MP600 Needlenose Multi-Tool.
Everything below is pulled live from the catalog, so what you see is what is actually in the Woodlawn warehouse today.