The 20, 30, 40, 60 and 90-numbers refer to an IC (internal Combustion) engine. A 60 (or more exactly .60) have 0.60 cubic-inches or 10ccm combustion chamber. A 2-litre motor is a 2000ccm motor. The numbers/class tells how big engine is required to make the plane or heli fly properly.

When it comes to electric motors the numbers vary a lot from manufactorer to manufactorer. All AXI-motors have numbers telling the diameter and length of the stator, and in addition the number of windings of copper on each stator... Thus a 2820/12 have a stator with 28mm diameter, 20mm length, and 12 windings on each stator. Compared to the 2820/10 which is equal except the numbers of windings the 2820/12 will have a lower kV because the wires are thinner to allow the stators to be filled, the /12 takes more voltage and spins larger diameter propellers, but at lower RPM. This makes the 2820/12 good for sailplanes and 3D, but the 2820/10 is better for pattern, with less diameter and higher pitch propeller. This give more speed, but less thrust...

When you choose a motoor for your plane you must first find how much power you need. Power = Watts! To fly a trainer you need 150Watt per kilo model. For 3D you need the double, 300 Watt per kilo at least. Then a suitable kV when you know the voltage your batteries chosen will have.

An example: you have built a sportplane weighing 2 kilos. You want to run it on a 4S LiPo battery, which give a voltage of appr 14 volt at load. 2 kilo model, needing 200 watt per kilo mean you need 400 watt at least. To get 400 Watt to a motor at 14 volt the amperes will be 400/14 = 29 amperes. You need a motor, battery and ESC able to deal with AT LEAST 29A. Add some margins...
If you look at AXI the 2820/12 will be a good choice on 4S LiPo. Add a 12x6 APC-E propeller, and you are at appr 33A and well within the "efficient" range for the motor.
Or buy any electric motor able to deal with 35A or more, with a kV in the 1000-area.