The linker is a tool one can use to only ship the minimal possible set of functions that a set of programs might require to run as opposed to the full libraries.
The linker analyses the intermediate code (CIL) produced by every compiler targeting the .NET platform like mcs, csc, vbnc, booc or others. It will walk through all the code that it is given to it, and basically, perform a mark and sweep operations on all the code that it is referenced, to only keep what is necessary for the source program to run.
For more details about how the linker works and other documentation please check docs/ folder.