When you are pledged to a faction, then you will be attacked by NPCs (and CNMDRs) belonging to rival factions, the rate you are interdicted by NPCs will increase noticeably.
You are free to go anywhere you like still, but if your rep with the power running a system is 'HOSTILE' then you will find they don't like you very much - for example as a Fed (Hudson) you enter an Imperial (Patreus) system - an NPC interdiction occurs as one of the patrolling imperial NPCs spots you - there are a fair few of these groups active in many systems, such as the Kumo Crew, Torval's Shield, Federation Agents, and so forth. The NPC tells you you are in the deep poo and opens fire....you defend yourself and kill him....congratulations, you will now have a WANTED tag where HOSTILE used te be, and you can only dock at outposts in this system (the wanted status is for that system only) as they tend not to have security - go to a Coriolis, Ocellus, or Orbis startport and the security will come for you, if the station doesn't get you first.
Killing NPCs or CMDRs from other powers will cause your reputation with those powers to drop - each power has combat actions that gain merits for pay and rank within the faction - and doing those combat actions will involve you killing NPCs, a good bout in an imperial system can drop you to 99% hostile in exchange for a few thousand merits. So if you were (like me) a Hudson pledge, and you go undermining in ALD space, you'll see the Imperial Reputation dropping quickly as kills are made. You can earn the rep back of course by trading and taking missions, but the drop is pretty meteoric compared to the rise, you might say.
You do not capture systems, your power undermine systems belonging to another power, and if the other power don't fortify it starts a rather protracted process where the enemy power first lose control over the system, and then it can be prepared for expansion, followed by actual expansion...there are several opportunities for a power to resist having a system taken from them, and also to stop it being gained by another.
You can deliver supplies of various types to help a power, and increase your rank (this is rank 1-5 in the powerplay faction, not the rank like 'Baron' and 'Post Captain' etc which are imporved via the bulletin board missions) - so if you are worried about becoming hostile to powers then you can avoid this by delivering thousands of tons of supplies....5400 tons, approx., per week to keep at rank 5 once you've got there. Delivering supplies avoids combat, and therefore avoids rep damage. (Barring the odd time you might get intercepted cutting acrosssomebody else's turf). This way yopu can stay pledged, gain rank within the faction, get the special weapons, and not end up with a bunch of Hostile tags.
If you unpledge I believe you lose the righr to buy more of the weapon, but get to keep any you have already on your ships....as I've not unpledged I can't swear to that though.
CGs don't count - being pledged in PP to a faction is what triggers the increased activity with NPCs.
Being Hostile, due to killing other factions' NPCs or CMDRs whilst in their own space, can be a medium sized bind as it rather prevents you using the large starports - an issue if you need large pads to land on. The increased NPC interdictions you are prone to can also be annoying, especially when you are getting a bit grinded out, just want to get back and dock and log out, and you get 3 or 4 serial interdictions from a variety of Adders, Sidewinders, and Vipers all convinced your Anaconda is going to be easy prey.
Dave
