Moving in HOTT Threat Zone

As Stevie says, I think the 'it' refers to the element, not it's base width area.

This makes sense if you see this point as being put in to allow for, and to clarify the paragraph above, which specifies the occasions when troops can pass over, under or through troops.

So, for example, an aerial could fly over an enemy element, through its threat zone, to it's rear, provided it ended up clear of the enemy's base, but it could not fly along the front of a line of enemy elements through their threat zones, as it wasn't flying over the unit.

At least that's the way I read it.

I too at first found the concept of Aerial movement and combat a little confusing.

Perhaps the best way of thinking about it is to realise that in HoTT all elements are
treated the same, and all of them generate and are affected by ZOC, but Aerial troops
have special interpenetration abilities (i.e. under or over).
It is also important to note is that ALL close combat involving Aerial troops, even air-to-air,
is done at ground level (allowing ground troops to also participate).

Here is a list of all the sections containing rules affecting Aerial elements:-
            Troop Definitions                 [page 6, section1 “Aerials”]
            Element AP Costs                [page 10, section 2 “AP Costs”]
            Bad Going for Aerials           [page 10, “Bad Going” section]
            Entering Woods or a BUA      [page 18, section 4 “Terrain Restrictions”]
            PIP Costs                             [page 15, section 1, first bullet point]
            Command Range                  [page 15, section 1, second bullet point]
            Movement Speeds                [page 19, section 2 “Movement Chart”]
            Aerials in Groups                  [page 15, section 2 “Groups”]
            Interpenetration                [page 16, section 2 “Passing Over/Under”]
            When in a ZOC                     [page 16, section 3, fifth bullet point]
            When in Water                     [page 18, section 3 “Crossing Water”]
            Contacting the Enemy           [page 17, section 1, last three paragraphs]
            Deploying Lurkers                 [page 14, section 1, third bullet point]
            Shooting at Aerials                [page 20, section 2, bullet point 2]
            Uphill or Water Bank              [page 22, section 2, fourth “Tactical Factor”]
            Bad Going Combat                 [page 22, section 2, bottom “Tactical Factor”, third bullet point]
            Combat Outcomes                 [page 23, section 2 “Outcome Chart”]
            Recoiling into Woods/BUA       [page 24, section 2, fifth bullet point]
            If Pursued                             [page 24, section 2, at the bottom of the page]
            Pushed Back Troops               [page 25, section 1, passing through, under or over]
            Fleeing Aerials                       [page 25, section 2, passing through, under or over]
See also the lower Diagram on page 35 about Aerials contacting but choosing not to fight,
yet are still able to overlap the enemy (elements P, O, L and J).

Some time ago, in order to make HoTT easier to understand, I made a “Pure HoTT 2.1 Crib Sheet”
that has all the various obscure and scattered rules all laid-out together in their relevant sections,
and this can also serve as an easy to follow play-sheet.
A copy of it can be found here:-
ibb.co/WPfbcTG

When Bob says ‘certain aerials’ I think he is referring to the following.

Page 16, section 2, “Passing, Over, Under, Or Through”, 4th bullet point:-
“Any ground troops can pass under enemy flyers or (enemy) aerial heroes
 or friendly aerials if these are not already in close combat.”

Note there is no mention of moving under enemy gods, dragons, or airboats,
so these prohibit movement ‘through’ them, even when not in close combat.

(In other words, any ground troops trying to move under these will be zapped
by lightning bolts, fiery breath, or dropped bombs, so cannot pass beneath them.
Exceptions: Sneakers and Gods can pass through any enemy, be they in combat or not)