The history and success of Microsoft Dynamics 365 Business Central's predecessor, Microsoft Dynamics NAV, starts with classic client.
From day one, this Visual C++ application was promising and after some decades of glory its retirement looks quite near. It will be replaced by the more modern, shiny, multi-platform, web application development-oriented Visual Studio Code.
Classic client used to be a two-sided application: a first-in-class production client for end users and at the same time a suitable workbench for developers.
With Microsoft Dynamics NAV 2013, it was deprecated as a production client and just simply named CSIDE (Client server integrated development environment), relegated to only a development application targeted as a Windows and/or web client.
