Preview and revert
Preview shows the result on the picked renderer and leaves the material asset alone. A change sticks only when you click Apply.
Preview
Preview is on when the window opens. Click Preview: ON in the toolbar to turn it off. That choice is remembered the next time you open the window.
Every material with Include checked on the Apply tab is previewed at once. Just like Apply to All Targets, every material uses the picked material's palette and material type. Only the base colour is each material's own.
Unchecking Include, or clicking Remove, restores that material alone. The other previews stay.
The channels you see match the apply scope at the bottom of the window. All channels shows every channel Apply would write. A narrower scope shows only that much. Preview never shows more than Apply would write.
The material asset does not change during preview. Picking another surface, closing the window, an assembly reload, entering Play mode, or the moment before a real write puts the original material back on screen.
Preview does not turn on a shader feature that is switched off. If the shadow is missing, turn that feature on in the shader and look again. How Apply can turn features on is on the Shading page.
Textures that Apply creates do not show in preview either. The lilToon Lite shadow colour textures and the gloss matcap appear after you click Apply.
Revert
Revert at the bottom restores the current material to the snapshot taken just before the latest apply from this window. If there is no snapshot, the material stays as it is.
Unity undo works too. Ctrl+Z undoes an apply, and Ctrl+Y does it again.