The squish (AKA “quench”) band refers to the area or areas,
where the piston comes very close to the cylinder head while passing through top dead center (TDC). ^1)
2)
Even though the fire has already been lit, these areas still contain unburned air/fuel mix as the piston arrives.
The piston sandwiches the air/fuel up next to the head, causing it to “squish”, and shoot out toward the flame front.
The result is more chamber turbulence, better atomization of the fuel, and the squish band even has a cooling effect on the fuel.
They result in more complete combustion (more power and better fuel mileage) as well as better resistance to detonation.
The squish band clearance would normally probably be in the .040“ to .060” range from the factory.
But when you put it together yourself, you can take some extra time and effort and optimize certain things.
Ideally you'd like it more on the order of .030“ (or .035” if you're using a full cast iron cylinder).

You can optimize the squish band by adjusting the height of the cylinder head relative to the piston.
(such that the piston come closer to the head)
This will give you a little free power, better mileage, and more resistance to detonation.
If you let it get too close, you can actually get contact due to things like thermal expansion and piston rock.

When 883 heads are used over the larger 3-1/2“ 1200 bore or 3-9/16” bore 1250 cylinders, a squish band is gained around the perimeter of the chamber.

You can use a piston height gauge to measure how far above or below the cylinder deck the piston is sitting at TDC.
Then add that measurement to the gasket thickness.
This method works fine with both decks being flat.
But it doesn't work well with heads that have angled domes.

You'll be placing some soft material, such as .065“ solder, onto the squish band area of the piston.
(clay has been used for this also but it's not as accurate as using solder which won't change thickness)
Rotate the engine through TDC, remove the material, and measure it's thickness.
It takes more time than measuring the deck as in above because it requires an extra disassembly/reassembly cycle of the cylinder head.
But this method works great with flat or angled head surfaces. The tools required are no more than if you are installing the heads.

This pictorial will guide you through the steps. It was done on an 883 to 1250 conversion piston, under an 883 head. An 883 head has a hemi chamber with no squish band, as described above, but it's chamber is only 3” in diameter to match the 3“ bore size of the 883. When 883 heads are used over the larger 3-1/2” bore size of a 1200, or 3-9/16 bore size of a 1250 as shown below, a squish band is gained around the perimeter of the chamber.