Flanking with flight

[UkrainianDM]

If two allies are attacking the foe from opposite sides, but one of the allies is hovering at ~5ft height from the ground, does it count as flanking or not?
Rules about flanking do not cover the height differences at all.

I tried to look for this situation on the forum but did not find any suitable topic.

[Castilliano]

The rules don't need to address this because there's no exception made for this situation (or it should've if there is!) Which is to say flanking or not then matters on the height of the target creature.

If it's Medium (barring any extraordinary shapes), then it wouldn't be flanked because one of the enemies is attacking from a corner edge so they aren't attacking through opposite planes.
This mirrors the situation where two people are (somewhat) on opposite sides, yet one's on a corner so isn't attacking through the opposite plane to the other (unless that one's on the opposite corner).

If the target's taller, so Large+ creatures (again barring extraordinary shapes), then it would be flanked since its "opposite plane" extends up into the air into the 2nd square (or more).

[UkrainianDM]

What if the hovering ally is a bird animal companion (small)?
Do you imply that an imaginary line should cross opposite edges of the cube in which the opponent is located?

[Taja the Barbarian]

Officially, you measure flanking in all directions:

Chapter 9: Playing the Game / Encounter Mode / Movement in Encounters / Cover / Special Circumstances / Three-Dimensional Combat wrote:

Source Core Rulebook pg. 477 2.0

In aerial and aquatic combat, you might need to track positioning in three dimensions. For flying creatures, you might use one of the following methods:

Find platforms to place flying creatures’ miniatures on.
Set a die next to a creature with the number indicating how many squares up in the air it is.
Make a stack of dice or tokens, 1 per 5 feet of elevation.
Write the elevation next to the monster on the grid.

In underwater combat, choose a plane to be the baseline, typically the waterline, the sea floor, or a stationary object you can measure from.

As with ground-based movement, moving diagonally up or down in 3-D space requires counting every other diagonal as 10 feet. Measure flanking in all directions— creatures above and below an enemy can flank it just as effectively as they can from opposite sides.

I take this to mean a 5' square becomes a 5' cube, so a creature threatening from your north and 5' above would require an ally threatening from your south and 5' below in order to actually flank you, like how attacking from the NE corner requires an ally in the SW corner to get the flanking bonus in a 2D battle.