The impact crits are more for non-solid/malleable force as opposed to a solid object colliding with a solid object. Hitting the ground from a fall: If the ground is solid, then definitely a Krush crit. If the ground is soft, like a bog/swamp/peat moss, then Impact would be more likely as having all the wind knocked out of you from the sudden/immediate stop, not so much from smashing into a large boulder.
Wave of water, wall of air, Gravity ball (Elemental Companion), Fire ball attack, Force Ball attack (Force Mage). A force that can pass around and/over the target (or the target can pass through the force), wave of air from an explosion would be example of impact. Having the wind knocked out of you very suddenly. The force could still break bones the way a Krush crit does if it is severe enough.
Mace crushing a skull, body falling onto a rock from a high height, sword striking chain mail armour would produce a Krush crit rather than a Slashing crit. All the force is focused along a fine edge to break a bone, but not necessarily to slice the arm off. An unyielding object such as a weapon or a rock is more likely to result in a Krush crit rather than an impact crit.