When deploying to a Linux stack, the mono runtime is bundled with your app (in app/vendor/mono) and used to run your application (via the detected container)
We try to always bundle the most recent Mono runtime (currently 3.2.5)
##Memory
The total available memory is specified when an application is pushed. The .net-buildpack uses this value to control the maximum amout of memory that Mono can use for the heap via the MONO_GC_PARAMS environment variable
MONO_GC_PARAMS="major=marksweep-par,max-heap-size=$(ENV['MEMORY_LIMIT']-48M)"
Note: if the total available memory is scaled up or down, the .net-buildpack does not re-calculate the memory settings until the next time the application is pushed (ie, staged).
Yes, 48M is completely arbitrary. Mono needs some memory for non-heap stuff, and this seems to be enough.