Comment by MrRadar

It's a pretty classic client/server setup (not too different from modern web apps, actually). The C code is the server and the C# code (which is actually a plugin to another application written in C#) is the client. The C server can run independently of the C# clients but isn't very useful (to our customers at least) by itself.

Most of the C# code's responsibility is taking user commands from the plugin's host application and converting them to messages sent to the C server for further processing, which isn't very speed/latency sensitive most of the time since it only has to operate at user-perceivable timescales and on small amounts of data. The remainder of the C# code is some custom UI controls and dialogs.

The network communications (if you squint hard enough) somewhat resembles REST with custom binary protocols instead of HTTP and JSON; if REST were in style when this code was first written (around 10 years ago) it almost certainly would have been used for the network/message layer instead.