Science & Industry

I'm not entirely sure what you're asking but I'll give a throw at an answer anyway.

1) first, you can set up your pos to attack anyone you want. Right click the tower and there is a menu. Look around. You'll find defensive settings. IIRC the default is to shoot anything that isn't blue.

2) Big guns have long range and can hit hard... but they're pretty useless against small ships with short range. If you think your POS might get shot by a fleet of large ships then you might choose different guns than if you think someone might try a drive-by shooting. The smaller the gun the more useful they are against BS sized ships and smaller.

3) On the whole, the best defensive approach to a POS that isn't likely to be attacked by a large fleet is what people call a "****" pos or a "dickstar". These are POS's that are well tanked with hardeners and use about 80% ECM modules, scrams and a few guns (google it for ideas for a fit). It would take a very substantial fleet a very long time to get through it because most of their fleet would be jammed most of the time and they would be shooting against very high resists. The idea is to bore the enemy to the point that they dont' think it's worth it.

Any of that come close to answering your question at all?

T-

The rules for when they engage based on standings depends on the system security (high vs low vs null) . I haven't run a pos in a while so some of this may have changed.

In high sec, only war targets can be targeted (and at one point you had to be in 0.5 or 0.6 to be able to manually target them period).
In low sec, only attackable entities (war targets or blinky criminals) can be targeted and attacked. Towers will auto target these entities as well.
In null, they will target anything you tell it to and with auto target/fire. Most people put their corp up to attack any entity with less that 0.01 standings (NBSI).

With that all said, the different types of batteries make a BIG difference, and each race have bonuses to each battery type.

All batteries use an ammo size one step bigger than the modules size, so small batteries use medium ammo, mediums use large, large batteries use XL ammo. You have to put the ammo in the modules themselves before you online (iirc). T1 or faction ammo is allowed. Note: T1 crystals will get used up, they are not infinite in pos batteries.

Okay, now how effective are they. Short answer, not very, and absolutely not effective unless you are manning them by real people. The batteries target random entities and rotate among the attackable entities on grid. This means an unmanned pos might only shot a ship once every couple minutes, once, then move to another target.

All missile batteries suck, horribly. They do almost no damage and require cpu, so go offline when the pos is reinforced. People tend to like to substitute Large ACs and Medium Arties cause they fit (cpu/power wise) and do pretty good damage.

ECM batteries can help you against an RR gang, along with Med arties / Med/LargeAC to hit BS.

tl; dr;
A pos can't defend itself, even manned. But if you are smal out how you lay things out around your pos, and how you man the batteries, it can give you an advantage during a fight on that pos. The main rational of having the guns there are so people can't spend ages hitting your pos in untanked ships without worry. Don't ever expect them to defend themselves and don't forget that you loose all cpu when reinforced, so make sure you have defenses that are just power (like gun batteries)

You could have just asked me.

Manage -> Defense : Sentry Gun Settings
* Attack if at war. [Always selected. Only setting that is valid in hisec.]
* Use alliance standings. [Usually selected when in an alliance.]
* Attack if standing below N [Usually set to 0.1 to attack neutrals and below.]
* Attack if other security status is dropping. [Huh? Totally useless.]
Note that standings do not apply to individuals, only corps and alliances.

Automated defenses are limited, but can still be a deterrent to small gangs trying to prompt a fight with an attack notification. Short-range turrets have a 90 km activation range, and long-range have a 300 km activation range. All EWAR has a 300 km activation range. Player max lock range is capped at 250 km.

When manually targeted with gunners, activation range does not apply, nor does the player lock range cap [this can surprise logi pilots that hang-back behind sniping fleets], and a POS can be quite effective vs. small to medium sub-cap fleets, or a few dreads. Beyond that though... well, start planning your escape!

https://forums.eveonline.com/default.aspx?g=posts&m=4946006#post4946006

Example what is possible with 11 POS gunners:
http://puu.sh/byzEg/3dfa719beb.jpg

U-W0T Brave POS Defends TCU's from PL and Provi
[Click on the title on Reddit to see a partial killboard.]

Battle Report:
S25C-K POS Defense AAR


Personally, I don't recommend using faction turrets, as they can attract unwanted attention.

Your guess is as good as mine. I've never tested it, nor have I seen a definitive explanation that I trusted.

The setting could be an attempt for defending a public "free-port".

Some of the POS settings were from legacy planned uses of POS, that were never completely implemented. Example: public facilities.

CCP removed several such settings, and I bet CCP just removed them from the UI code and they probably still exist behind the scenes in the POS code. [Might have something to do with POSes that go Skynet for no reason.]