What does "Summoned creature has maximum hit points" actually mean?

Question

I'm running Princes of the Apocalypse for my group, and they're about to encounter their first elemental prince, Yan-C-Bin (p. 220). I see that he has the following ability:

Summon Elementals (1/Day). Yan-C-Bin summons up to three air elementals and loses 30 hit points for each elemental he summons. Summoned elementals have maximum hit points, appear within 100 feet of Yan-C-Bin, and disappear if Yan-C-Bin is reduced to 0 hit points.

The part I'm confused about is "maximum hit points." Air elementals have their hit points listed as "90 (12d10+24)" in their stat block.

Is this line meant to make sure people are deploying a fully-healed air elemental as opposed to one with 30 HP (regardless of whether they use the average hit points or they actually roll their hit dice), or is it meant to convey that these are extremely robust air elementals and each should have the maximum possible hit points as decided by the hit dice, 144 (120+24), and the fully-healed part is implied?

Answer 1

It means that the elementals will have 144 Hit Points

If the designers instead wanted the air elementals to have rolled or average hit points they could have said:

Summoned elementals have their normal hit points

But instead they've specified that they have "maximum hit points". This comes down to an English reading then, and to me this would mean the elementals have "the highest hit points they normally can have" which, as you've pointed out, is 120+24 or 144.

Answer 2

The latter

Summoned creatures are always assumed to be summoned at full health, unless when mentioned otherwise. The hitpoints entry in the statblock assumes that the creature has rolled the average on each hitdice it has.

This specific monster has an ability that changes that, and instead summons versions of that monster that rolled the maximum on every dice