In paint tool SAI, just above the layers and below the opacity bar there are three check boxes: Preserve Opacity, Clipping Source, and Selection Source. Preserve opacity locks the layer's opacity, causing you to only draw on what's already on the layer. Clipping source allows you to only draw on what's on the layer below that layer (if there are two or more layers set to this in a row, it draws on the first layer beneath all of them.) I'm not entirely sure what selection source does because I have never used it, though I assume it makes layers with it on become the only selectable part of the canvas.
Hope that helps.
In paint tool SAI, just above the layers and below the opacity bar there are three check boxes: Pres
