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Using Warp Spiders and a Ruin as examples.

Warp Spiders have Infantry and Fly keyword

Ruins is a type of Terrain Feature.

Rulebook indicates Infantry may move through Ruins (Terrain Feature) as if they are not there.

Rules Commentary (pages 8-9) describes moving up and over 'Terrain Features'.

How do the Rules/Rules Commentary interact? Do Warp Spiders move through walls of a ruin or does one measure vertical distance?

To me, the Ruins Terrain Feature specifically gives an exemption for the Infantry keyword allowing Warp Spiders to move directly through the Ruins as if it were not there (assuming moving along the bottom to floor and remaining on the ground throughout the movement).

Warp Spiders would ignore Ruins as they are still Infantry, even with the Fly keyword.