When you are tapping, especially on hard materials, you start off taper, second then plug.
The difference in these being the amount of lead on the tap but once you are past the lead you are up to full OD no matter what number tap it is so all the work is being done by the lead.
Doing some stainless today and just thought that i have not seen the taps I use posted on here.
Don't know if they have a special name or not but I call them serial taps. Still taper, second and plug but as well as the difference in leads the OD is also reduced on the taper and second so it reduces cutting forces.
You have to use them in order as the taper or second will not finish a thread.
I have tipped these so you can see the flats on the crests of the threads showing the reduced OD.
I get these for a good tool company in China, not expensive but not cheap rubbish either.