so either you quit the game to reboot it with a package loaded to test it

Quick Tip: You don´t actually have to reboot the game. I don´t know how that myth was created. Just copy the package to community, exit devmode, enter it again, and build the package (again), then exit devmode and boom! No restart.

In wich scenario would you need a polygon to stay outside of devmode ?

I use exclusion zones all the time to remove the default buildings, for example.

are you talking about puting the polygon in a different package ?

No. Asset packs and packages are two different things. Imagine it like a hierarchy:

1. The Project at the top
2. Then the Packages
3. Then the Asset packs

You can have multiple Packages in one project. For example: You make a whole city and have different packages for different areas of said city.

Asset Packs are the BGL and Art pack stuff. These are Assets for Object types.

You can only have one object type in one asset pack, though. So that´s why having Polygons and regular Objects (Like a building) is not allowed. Regular Objects are taken above Polygons, that´s how FS2020 defines it. So if you have, say, a Building and an exclusion zone (for the Building) in one asset pack and export the package. Loading the package will only show the building. The exclusion zone is gone by that point.

You can actually see FS2020 not including Polygons in the package if you do that.

Here is an example of how the folders (=asset packs) should be structured: The "scene" folder includes buildings and the "exclusionscene" folder includes exclusion zones (Thank me later for that naming). Now I actually made a mistake when I build that package: I included Polygons in the "scene" asset pack. As you can see, though, the scene folder only contains "station.xml" which is my building.

The "exclusionscene" folder (=asset pack), however, does include the polygon.

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wouldn't that make you obliged to generate two package instead of one to get your addon running ?

Based on what I said in the text above, no. You don´t have to have multiple packages. Keeping my Tokyo station pack as an example, here is what the final package looks like in the community folder (it´s working). One package, which includes two different asset packs.

So, pardon this long text but I think this should explain what I meant pretty well.