What do type qualifiers __I and __IO mean?

[Tsan-Ming]

I am working with the nRF51 Software Development kit for the nRF51822 Evaluation kit There are some kind of type qualifiers such as __I, __IO appearing in the header, e.g., .\Keil_v5\ARM\Device\Nordic\nrf51822\Include\nrf51.h. What do that mean? Where can I find their definitions if there are? Can I omit them?

Thank you for your help.

[kirito]

(core_cm0.h)

#ifdef __cplusplus
  #define   __I     volatile             /*!< Defines 'read only' permissions                 */
#else
  #define   __I     volatile const       /*!< Defines 'read only' permissions                 */
#endif
#define     __O     volatile             /*!< Defines 'write only' permissions                */
#define     __IO    volatile             /*!< Defines 'read / write' permissions              */

[Pavan kumar]

Have a question, __O and __IO both macros defined for same volatile how one will be write only and second one is read/write. How its possible can you please explain.