[G] Unit/Structure Selection Priority

Priority from highest to lowest. High priority = small number.

ALL
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Terran Units
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Zerg Units
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Protoss Units
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Terran Structures
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Zerg Structures
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Protoss Structures
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This is what the word "comprehensive” requires in my standard. Let me know if I missed something. Bolded = structure. The only ones I didn’t test were vespene geyser, xel’naga tower, scantipede, Artosis bot 2000 etc. because they are somewhat tangent neutral objects. Well, no campaign-only diamondback etc. either.

The list alone might not be self-explanatory enough.

No Exception
Priority level is universal. When A<B and B<C, then always A<C. No matter how many different units/structures are selected in whatever order, priority follows the list consistently.

Tab
Pressing tab while having selected multiple types of units/structures cycles through them and allows you to use spell/ability/production from units/structures that don’t have the highest priority. Many Terran players use this to cycle through barracks-factory-starport while having them all in a same hotkey rather than 3 different ones. Also, some Protoss players press tab once to blink stalkers because stalker has the second highest priority after sentry in the deathball without HT/mothership.

Tab-selected units=stalkers are highlighted.

Same Priority
As obvious as it sounds, units/structures of the same type have exactly the same priority. However, I do not know how they are ordered among them at the bottom of the screen. Order of birth seems irrelevant. This randomness(?) is responsible for Zerg units made from a undesirable larva at a distant hatchery when all hatcheries are hotkeyed together. All larvae have the same priority, but the first larva could be from your main or 3rd depending on… purely luck(?)

These 24 marines have the same priority, but what determines who fits where among them?

Unit vs Structure
Generally speaking, units have higher priorities than structures when selected together, but there are a few exceptions.

Why?
It seems that priority is determined primarily by "SubgroupPriority value" each unit/structure has. Open map editor -> Data -> Units in XML view for

Ordering some with this value,
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Not a single unit/structure contradicts actual test results in-game. Other factors must exist for tie situations, but this SubgroupPriority value is the main contributor for sure.

Mode
Siege tank has tank mode and siege mode.
Viking has fighter mode and assault mode.
Warp prism has transport mode and phasing mode.
Ghost has weapons free state and hold fire state.
However, all these are irrelevant when it comes to priority.

Again, I’m not sure how order within the same unit type is determined.

Barracks/Factory/Starport
Add-on attached is irrelevant. Reactor – regular – tech lab order is consistent at the bottom of the screen, but it plays no role in changing priorities.

Pressing tab once, all factories are highlighted together regardless of different add-ons.

Tech Lab and Reactor
Tech labs’ priorities depend on the main structures they are attached to. On the other hand, all reactors have the same priority despite different main structures.

Lifted Structures
Once lifted, these structures no longer have the same priorities as the landed ones until they land again.

Supply Depot
Lowered supply depot literally has a lower priority than raised supply depot by a large margin within Terran structures.

Spine/Spore Crawler
Just like raised/lowered supply depot, uprooted and rooted crawlers have different priorities.

Creep Tumor
Active creep tumor or not is irrelevant.

Changeling
Normal/marine/zergling/zealot form changelings all have the same priority.

Broodling
Whether it is spawned from a broodlord or a dead structure is irrelevant. It is kind of obvious, but I tested just in case.

Cocoons
Yes. Even cocoons have priorities. Not surprisingly, cocoons are at the bottom of the list with infested swarm egg being the lowest of all. Blizzard can't nerf it any further

Neural Parasited Units
NPed enemy units are nothing special. They follow the list, too as if they were your own or your teammate’s units from beginning.

Burrowed Units
Burrowed units have slightly lower priorities than their fellows on ground.

Hallucination
Unlike burrowed units, hallucinated units have, interestingly, higher priorities than real units of the same type. Not that it matters for gameplay ever, but who knew?

Dark Templar
Dark Templar has 2 different graphics: with dagger or with double scythes. However, this doesn’t affect the priority at all.

Without bothering to use tab key, your marine/siege tank/zergling/baneling/zealot/stalker/sentry army would use “explode” (or burrow) because, unfortunately, baneling has the highest priority among them. I think many team game players have this experience where you couldn’t stim marines or place forcefields because of a few banelings your teammate left behind or secretly shared control of. (Never share control without telling your teammates about it for this very reason.) I had a similar experience in a team game, and the liquipedia page was incomplete, so I decided to take it into my own hands. Even for 1v1, structure priority part must be new information. I don’t think knowing this priority suddenly makes anyone play any better, but I hope it’s been an interesting and somewhat helpful read. Feedback/correction is much appreciated.