For historical reasons, the posix standard has classified several builtin commands as special. When Bash is executing in posix mode, the special builtins differ from other builtin commands in three respects:
When Bash is not executing in posix mode, these builtins behave no differently than the rest of the Bash builtin commands. The Bash posix mode is described in Bash POSIX Mode.
These are the posix special builtins:
break : . continue eval exec exit export readonly return set shift trap unset