These settings are available if you have Any Platform or Standalone selected in Platforms tab.
Note: These checbkoxes affect CPU values only, that’s why [[PluginImporter.GetCompatibleWithPlatform]] won’t reflect any changes when clicking on the checkboxes, instead use [[PluginImporter.GetPlatformData]], pass CPU for key. You can also check *.meta file to see exactly how values are setup, for ex., if your plugin is named as Plugin.dll, search for Plugin.dll.meta.
There are two types of plugins:
CPU specific plugins, for ex., native plugins which can be used only for specific process architecture - 32 bit or 64 bit.
Plugins working with any process architecture, for ex., managed plugins.
Native x86 dll file
because native dll files are only usable on Windows, you only need to weak Windows settings
x86 dll files cannot be used in x64 process
keeping above thing in mind, check Windows x86 checkbox, leave others unchecked
Managed dll file, also know as assembly file
if assembly was compiled with Any Cpu option and targets .NET 3.5, all platforms (Windows, Linux, OSX) can use it
check all the checkboxes, and select Any CPU for universal targets
Bundle folder, containing x86 library
bundles are used by OSX platform
check OSX x86 checkbox, and pick x86 of Universal
Bundle folder, containing x86 and x64 libraries
check OSX x86 and x64 checkboxes, and pick Any CPU of Universal