Infantry gear works as follows:

Militia: Baseline stats, but with higher fitting cost or slightly lower PG/CPU capacity on the suit. Significantly reduced cost.
Standard: Same baseline stats as Militia, but with better fitting capabiltiies and a significantly higher pricetag
Advanced: A step above Standard in many aspects.
Prototype: Another step up from Advanced.

Vehicles work as follows:

Militia: Baseline stats, short on fitting capabilties,but really, REALLY cheap.
Standard: N/A - closest is the real-money pack that comes with 40 "Militia" tanks that have higher PG/CPU capacity than the actual Militia HAVs. - Have to be bought tho with real money, so for me doesnt count because ther i not really an ISK equivelent
Advanced: Notably higher stats than the Militia baseline, and better fitting capabilities than the "real money" Militia tanks. AUR variants have even better PG/CPU capacity. - No they dont, AUR Madrugar is the same as ISK version, only the AUR gulogi has 50 more CPU than the ISK equivelent, its not even a side grade plus it doesnt require any skills to use, for me its below the normal gunlogi and madrugar
Prototype: Better stats all around again, and on top of the better stats, a unique role-specific bonus - so far this only means Marauders with damage buffs for specific weapon types, but presumably other Proto HAVs will come with different advantages.- Barely better stats plus the diff between the basic tanks and marauders is 1 extra slot in the wrong area, armor tank extra high slot which is next to useless, the shield extra low slot is more use for the shield, these are not proto in my eyes

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I'd say we have a valid Prototype-level tank tier already. We just need a "tank" version to go up against the current "DPS"-focused Marauders. Combine that with some alterations to how the game works in relation to the tank vs. tank and AV vs. tank situations, and we should see a lot more interesting vehicle fits.

According to the game itself, we don't have a Standard tier.

Sica/Soma = Militia
Gunnlogi/Madrugar = Advanced
Sagaris/Surya = Protoype.

Also according - as I showed in my post - to the logic that applies to most Dropsuits, we don't have a Standard tier. Heavies being the only exception where there's a non-fitting difference between Militia and Standard.

Look at how the stats line up, it makes sense that a "Standard" HAV should have the same shield/armour HP, and the same of every other stat EXCEPT PG/CPU (and maybe slots) when compared with the Militia variant. Much like the PS Store "Militia" HAVs are, but with skill requirements to purchase/use, and a higher ISK cost. The Store versions would then be the equivalent of an AUR Standard item, making them cost real money, but saving you on skill training to get that extra fitting capacity. Just like AUR gear does for infantry. It doesn't make sense for the Madrugar and Gunnlogi, with large increases in PG/CPU, slots and both shield and armour HP to be "Standard" - they improve too many aspects to fit the established pattern.

Unless, as has been suggested multiple times, the current "Marauder" Proto tanks are the "DPS" variants, and we're also getting "tank" variants with built-in damage resistance or regen instead of the damage buff the Sagaris and Surya get.

The current Proto HAVs might be glass cannons by design, and the more durable variants are in the pipeline for future builds.