How do the air dead spots work in Starcraft II?

In Starcraft 2, ground units can only see things on elevations equal or lesser to the elevation they are themselves. Air units can see things regardless of elevation.

An air unit, like an overlord, above one of those pillars is on an elevation greater than the elevation of any nearby ground units. Since there is usually no nearby location that is equal or higher elevation to the pillars, the only way to see the air unit is to bring another air unit nearby.

These pillars are just small areas of higher elevation. They're also impassible to ground units, so you can't plop down a siege tank up there. They have no other special rules.