1. Flightbeam recommends value between 15-21. May I know if 15-21 is also recommended if I also have KLAX v2 installed? I worry that such value is good for KSFO HD but detrimental for KLAX v2
We designed our sceneries with a setting of 1, that's why it's the default value.
Every other value, it's your own preference. Everybody has a different idea of what it's best, either a faster fps with less chance of OOMs ( small values ) or a smoother flight with a lower fps and higher chance of OOMs ( higher values ). You just have to try with the settings that works best for your system and your personal preferences.
2. Actually what is the meaning of the number. I have read other relevant posts explaining that the paramenter can control the number of small objects able to pop-up. But what is meant by 1 and what is meant by let's say 21?
It's a multiplier. If an object
handled by the Addon Manager was designed to have a loading range of, let's say, 0.5 NM, if you set the Anti-pop up to 4, the actual loading range will be 2 NM.
A very high value will just load everything at the same time, at the maximum scenery loading range, which usually changes from 10 to 16 NM, depending on the scenery.
Of course, if you set a very high value, there will be a LARGE pause with you enter the scenery loading area (because everything will be loaded at once), but no other pauses. Which will of course probably cause a lower average fps and increase the risk of OOMs so, it's either many brief pauses with faster fps and less chance of OOMs, or one big pause with lower fps a smoother flight and more chances for OOMs, there's no free lunch.
Some users mistakenly associates the stutters to the usage of the Addon Manager, which is obviously not the case. It's just that the default value of 1 is one that keeps the highest fps with less chances of OOMs.
A scenery made without the Addon Manager, simply means you have NO CONTROL over it, and the situation is very similar AS IF the Addon Manager had the Anti-popup setting to its highest value, which means everything will be loaded at the same time, with no memory handling or optimization (yes, even if the scenery has LODs, it will load the object in memory anyway, even if you don't see it), and the "big pause" it's just the same, but you don't get to notice it, because it's hidden by the "Loading scenery" progress bar.
However, if I want everything in fsx.cfg unchanged, but change the value of Antipopup, I still need to click Save to FSX button for applying the Antipopup?
No, in case of the Addon Manager-specific settings, the value is used immediately, so you don't have to Save the settings. Of course, it won't be saved for the next session, but if don't want to change the other tweaks in the FSX.CFG, you don't have to Save it.
4. Actually where does the value of Antipopup save to? If in the future I deleted or change the other fsx.cfg, will the settings under Addon Manager Tweak be changed?
The [fsdreamteam] section of the FSX.CFG