Symbol is not to be dead stripped

The N_NO_DEAD_STRIP bit of the n_desc field only ever appears in a relocatable .o file (MH_OBJECT filetype). And is used to indicate to the static link editor it is never to dead strip the symbol.

Symbol is discarded

The N_DESC_DISCARDED bit of the n_desc field never appears in linked image. But is used in very rare cases by the dynamic link editor to mark an in memory symbol as discared and longer used for linking.

Symbol is weak referenced

The N_WEAK_REF bit of the n_desc field indicates to the dynamic linker that the undefined symbol is allowed to be missing and is to have the address of zero when missing.

Coalesed symbol is a weak definition

The N_WEAK_DEF bit of the n_desc field indicates to the static and dynamic linkers that the symbol definition is weak, allowing a non-weak symbol to also be used which causes the weak definition to be discared. Currently this is only supported for symbols in coalesed sections.

Reference to a weak symbol

The N_REF_TO_WEAK bit of the n_desc field indicates to the dynamic linker that the undefined symbol should be resolved using flat namespace searching.

Symbol is a Thumb function (ARM)

The N_ARM_THUMB_DEF bit of the n_desc field indicates that the symbol is a defintion of a Thumb function.

The N_SYMBOL_RESOLVER bit of the n_desc field indicates that the that the function is actually a resolver function and should be called to get the address of the real function to use. This bit is only available in .o files (MH_OBJECT filetype)

The N_ALT_ENTRY bit of the n_desc field indicates that the symbol is pinned to the previous content.

The N_COLD_FUNC bit of the n_desc field indicates that the symbol is used infrequently and the linker should order it towards the end of the section.