BE0070: Differing framework types were detected in the documentation sources
Error BE0070: Differing framework types were detected in the documentation sources (i.e. .NET 5 and .NET Standard 2.x). Due to differences in how the core types are redirected, the different frameworks cannot be mixed within the same documentation project.
Cause
Visual Studio solutions or projects were included as documentation sources that contained projects for a combination of the full .NET Framework, .NET 5.0, .NET Standard, Silverlight Framework, and/or the .NET Portable Framework. This is not allowed as each uses separate framework assemblies that cannot be mixed as it would cause incorrect reflection information to be produced for the combined sets of classes.
Solution
If the documentation source solutions and projects use multi-targeting, you can set the target framework to use explicitly by specifying it in the TargetFramework property on each documentation source.
If the projects do not share a common target framework, separate the projects by platform into their own help file builder projects. The help file projects can then be built separately to produce two or more help files. If you would prefer to have one help file for all of the assemblies, use the Version Builder Plug-In to include the other help file projects into the primary project.
Typically, projects for the full .NET Framework and .NET Standard 2.x can be mixed in the same help file builder project as can projects for .NET 5.0, .NET Standard 1.x, and .NET Core 1.0 through 3.1. All other combinations will usually have to be separated. There may be combinations that have not been discovered yet that are incompatible. These will result in incorrect documentation or an error that causes the MRefBuilder tool to fail.