The conflict checking is only between addons, so if you have a campaign with custom skins and then a survivor skin pack you'll be able to see that they will conflict if you have the skin pack active while playing that custom campaign. (No that that's necessarily a bad thing, but it can help point out why you may not be seeing exactly what you expect)

We don't report conflicts between addons and the base game because that's likely to be true for almost every addon you install, and it's usually intended that the addon override the base game.

So, one more time. If i place like new skins and models in addon tagged "campaign", then this skins and models will work only inside this campaign. Right? And if i place skins or models in addon tagged "Survivals" or "Infected", then this skins and models will work in the entire game, except for the custom campaigns, that have replacements for same content. Right?

Also can scripts from one campaign conflict with content from another, like population.txt, soundscripts, event scripts etc.?

So main question is which of campaign's content only works inside this campaign, and which affects the whole game?

Yes to be sure like 100_o/o_f@ke said. For scripts or models can it conflict with another campaign or even when the player is not playing effect the Stock gameplay(scripts ect)

The tags are just used for reference. If an addon has a mission script, then it's considered a campaign addon and will only be loaded when you're playing that campaign. If it doesn't have a mission script then it's considered pure content and can be loaded at the same time as any other addons.

So we report conflicts between content and campaign addons, and between content and content addons. Scripts in one campaign addon shouldn't conflict with scripts in another campaign addon, since only the one you're playing is loaded at that time. Similarly, scripts in a campaign addon will override the base game only while you're playing that campaign.

The general answer is, all the rules are the same as they have been for addons since the game shipped, so all existing content on l4dmaps or other sites will behave the same way it always has. Workshop is primarily just an alternate delivery method.

Originally posted by Kerry:

Similarly, scripts in a campaign addon will override the base game only while you're playing that campaign.

I still don't think this is true.

I have modded talker scripts for each character in my campaign and when I just played a stock campaign I can issue a custom concept rule command and it fires and speaks the lines in dead center as its modded in my campaigns scripts...

So what is going on there??

I never even loaded my campaign up to play i just started l4d2 and created a dead center local lobby..

If you want to test it start a stock map (map c5m1_waterfront) with my campaign downloaded to your system.

Then issue the command: ent_fire director fireconcepttoany MTEllisStories

MTEllisStories is a custom concept line added to the talker scripts(thanks to hrcspkla for the scripts)

Originally posted by Kerry:

If an addon has a mission script, then it's considered a campaign addon and will only be loaded when you're playing that campaign.

Gnome's model is replaced in some campaigns and, when I have one of this campaign installed, original gnome in Dark Carnival has collision model of the custom gnome model from last installed campaign: http://puu.sh/1LnGm
Also gnome collision is broken in early installed custom campaigns that replaces gnome model.
So I just intersted why it happens?

More serious problem is caused by custom campaigns that have "soundscapes_manifest.txt" in their VPKs. When another custom campaign with its own soundscapes starts, wrong soundscapes are played on the first map, but after level once changes all soundscapes in current campaign work right.
So one campaign with "soundscapes_manifest.txt" in VPK affects to all other custom campaigns that have their own soundscapes.

For example: http://puu.sh/1Lows
This is from comments to Undead Zone campaign, that have its own soundscapes.

The campaign with "soundscapes_manifest.txt" that I know: Vienna Calling 2, Silent Hill L4D2.

There are some manifests (such as the particles manifest) that get processed before the inactive campaigns are unloaded - but that issue has always existed in Source and is a technical limitation of the engine. If you install those campaign addons in the current public version of L4D2, you should see the same behavior. Unfortunately, there's no good way to fix those in this engine, which is why when the game is doing something strange the first step is always to disable addons. If you see addons behaving differently in the beta than they do in public, then that would be a bug.