Alpha lock is a function that lets you partially change the color of the line drawing in the layer that has been opacity locked with a brush.
Although the same expression can be done using the clipping function, no additional layer is necessary with alpha lock.
Prepare an illustration.
Open ①the Layout window, select ②the layer with the line drawing, and tap on ③Alpha Lock to turn it on.
The color sticks out of the drawing with the alpha lock off...
But if you turn it on, the color stays within the drawing.
Using the airbrush in the Brush tool can let you create gradations, which will add a unique impression to your illustration.