buffer ake c64 memory map curiosities

[mvdhoning]

why is the buffer for c64 target positioned at 0800 and what is it for?
why does the code start at $0900?
also i do not seem to be able to set code for the c64 target on compiler command line?
found that buffer and codeorigin are set hardcoded at Targets.pas.
just for fun i tried changing buffer to $c000 and codeorigin to $0810 and sure it seem to work fine and give a smaller executable.
but before i pursue further into this rabit hole i hope the experts here can give me some insight on what the buffer is for (and why it is placed where it is)
thanks in advance
i wish both buffer and code origin could be set by compiler param

[tebe6502]

-code:<address>   Specify the memory start address for the code

-data:<address>   Specify the memory start address for variables and arrays

-stack:<address>  Specify the memory start address for the software stack (64 bytes required)

-zpage:<address>  Specify the memory start address of variables in the zero page (26 bytes required)

[mvdhoning]

i know that those commands exist. but do not seem to work for me on c64 target also none refer to buffer
changing compiler is the work around for me now as i can also disable the default charset change there.