Before you can use .NET libraries in your AL code, you must configure the project to allow them, by updating the target property in app.json. It must be set to internal in order to use .NET variables.
The next step in using .NET libraries is that you need to define a DotNet package. This lists the assemblies and types that you will be using. It's recommended that you create only one DotNet package and list all the assemblies and types in the single package. By default, you can only reference the mscorlib library in the package, but you can configure the AL Language extension to look at other paths for additional assemblies.
Once you define the DotNet package, you reference the assembly type by creating a DotNet variable and referencing the alias name you defined in the package.
You can only make use of .NET variables in on-premises environments. You cannot publish an application to the Business Central cloud platform if it contains .NET variables, so make sure that if you do use them in your on-premises solutions, you are aware that the solutions will not be able to move to the cloud platform as is.