Unity supports a number of .NET profiles. Each profile provides a different API surface for C# code which interacts with the .NET class libraries. You can change the .NET Profile in the Player Settings (go to Edit > Project Settings > Player) using the Api Compatibility Level option in the Other Settings section.
The legacy scripting runtime supports two different profiles: .NET 2.0 Subset and .NET 2.0. Both of these are closely aligned with the .NET 2.0 profile from Microsoft. The .NET 2.0 Subset profile is smaller than the .NET 4.x profile, and it allows access to the class library APIs that most Unity projects use. It is the ideal choice for size-constrained platforms, such as mobile, and it provides a set of portable APIs for multiplatform support. By default, most Unity projects should use the .NET Standard 2.0 profile.
The stable scripting runtime supports two different profiles: .NET Standard 2.0 and .NET 4.x. The name of the .NET Standard 2.0 profile can be a bit misleading because it is not related to the .NET 2.0 and .NET 2.0 Subset profile from the legacy scripting runtime. Instead, Unity’s support for the .NET Standard 2.0 profile matches the profile of the same name published by the .NET Foundation. The .NET 4.x profile in Unity matches the .NET 4 series (.NET 4.5, .NET 4.6, .NET 4.7, and so on) of profiles from the .NET Framework.
Only use the .NET 4.x profile for compatibility with external libraries, or when you require functionality that is not available in .NET Standard 2.0.
Unity aims to support the vast majority of the APIs in the .NET Standard 2.0 profile on all platforms. While not all platforms fully support the .NET Standard, libraries which aim for cross-platform compatibility should target the .NET Standard 2.0 profile. The .NET 4.x profile includes a much larger API surface, including parts which may work on few or no platforms.
Managed code plugins compiled outside of Unity can work with either the .NET Standard 2.0 profile or the .NET 4.x profile in Unity. The following table describes configurations Unity supports:
|API Compatibility Level:|
|.NET Standard 2.0||.NET 4.x|
|Managed plugin compiled against:|
|.NET Core||Not Supported||Not Supported|