FIL.NET is a part of the FabImage Library product. Its FilNet.dll and FilNet.TS assemblies are a .NET wrappers for the native FIL library. They contain all the functionality that native FIL library has, including types, functions and environment (hardware) settings.
Getting Started with FabImage Library .NET
Relation between FIL.NET and FIL/C++
HMI Controls for FIL.NET
FIL.NET Performance Tips
Code Migration Guide to version 4.11