As Jacob said, if the plane has a two-sided front nicker - either a single blade with two sharpened 'ears' or two blades mounted either side of the body, then it is intended to produce a trench across the grain, (or dado). Some rebate planes, a Fillister or Rebate plane just have a single nicker on the starboard side and usually an adjustable fence- a look-alike plane but a different tool.
If you can get your hands on a copy of R A Salaman's dictionary of Tools, he has an illustrated description of each type and its uses.
https://www.amazon.co.uk/Dictionary-Woodworking-Tools-R-Salaman/dp/1879335794
On a dado plane, this entails getting and maintaining each side of the front nicker to exactly the same height, each side with exactly the same edge geometry and the cutters have to be exactly the right distance apart which is exactly the width of the main cutter...... Derek Cohen's note that you found above explains very well the faff involved in getting all this right.
The knack (and main difficulty) is to get these things to do what you want it to do - that is to pre-cut two clean edges and remove a cross-grain shaving from one side of the plank to the other on each stroke.
One thing that works well is to sharpen both edges of the nicker, front and back, and to draw it backward toward you at the start of the stroke once or twice to sever the cross grain members before pushing in to a cutting stroke.
If your work doesn't entail a clean cut edge, then it's a good tool to use this way. However, all the tools that I've seen with front end nickers tend to tear out the wood rather than to give a clean cut. Getting the nickers to a sharp dependable edge is difficult. On this basis I'm never yet convinced that this type of tool was ever intended to do all of that in one stroke and leave a shatter-free edge.
A roughing out tool? Probably. Finishing to an acceptable meeting edge designed to be seen? No, probably not.
There are other ways - admittedly not as quick - to get clean sharp dado/trench edges with hand tools.