What does a pipe in a macro signify?

Question

How is the following macro definition resolved?

#define EMAIL_SERVER_ADAPTER_FATAL_ERROR     MSB_RETURN_TYPE_FATAL_ERROR | 1

I mean, is it resolved to 1 or to MSB_RETURN_TYPE_FATAL_ERROR and why?

Answer 1

| has no special meaning in macros. The macro is resolved to

MSB_RETURN_TYPE_FATAL_ERROR | 1

which is bitwise OR of two values (MSB_RETURN_TYPE_FATAL_ERROR and 1).