Loader is the part dedicated to manual loading of a PE. It is automatically added to your executable during the process of conversion (shellcodification). So, when you use pe_to_shellcode, two main things are done to your exe:
- the loader is appended
- the header is modified, to redirect the execution to the loader - thanks to this, after the conversion the PE can be injected and executed starting from its beginning.
Loader is built separately from the main executable.
- Buiding requires masm_shc.exe.
- More details described here.