Prevention effects are very similar to replacement effects, but are specifically related to preventing damage, and apply after replacement effects. For example, Tufted Turtle is a prevention effect. When applying damage to a Tufted Turtle, first apply any replacement effects (e.g. Lance, Critical Strike) and then apply the Turtle's prevention effect last. This rule ensures that prevention effects function properly. E.g., you can't somehow apply a Lance after the Turtle's prevention effect was applied, somehow dealing damage, which would be wrong. For more details, see the 'Damage' codex entry and subsections.