After MacOS X 10.1 when a new load command is added that is required to be understood by the dynamic linker for the image to execute properly the LC_REQ_DYLD bit will be or'ed into the load command constant. If the dynamic linker sees such a load command it it does not understand will issue a "unknown load command required for execution" error and refuse to use the image. Other load commands without this bit that are not understood will simply be ignored.
LC_SEGMENT = 0x1
segment of this file to be mapped
LC_SYMTAB = 0x2
link-edit stab symbol table info
LC_SYMSEG = 0x3
link-edit gdb symbol table info (obsolete)
LC_THREAD = 0x4
thread
LC_UNIXTHREAD = 0x5
unix thread (includes a stack)
LC_LOADFVMLIB = 0x6
load a specified fixed VM shared library
LC_IDFVMLIB = 0x7
object identification info (obsolete)
LC_IDENT = 0x8
object identification info (obsolete)
LC_FVMFILE = 0x9
fixed VM file inclusion (internal use)
LC_PREPAGE = 0xa
prepage command (internal use)
LC_DYSYMTAB = 0xb
dynamic link-edit symbol table info
LC_LOAD_DYLIB = 0xc
load a dynamically linked shared library
LC_ID_DYLIB = 0xd
dynamically linked shared lib ident
LC_LOAD_DYLINKER = 0xe
load a dynamic linker
LC_ID_DYLINKER = 0xf
dynamic linker identification
LC_PREBOUND_DYLIB = 0x10
modules prebound for a dynamically linked shared library