The answer to the question is actually the combination of comments from @CommonsWare and @Chris. So, for the sake of progeny, I am going to consolidate the comments and make it available in one place.
Basically, there are two predefined constants for the requestCode and they are Activity.RESULT_OK and Activity.RESULT_CANCELLED. However, android developers can also set custom codes for their apps by using the offset Activity.RESULT_FIRST_USER. Doing so ensures that there are no clashes between constants set at the OS level and the app level.
Purely, my opinion, I think that the FIRST USER suffix is meant to refer to developers – just like how end consumer refers to consumers of a said product – who are the first users before the app users.
Below is an example of how you can use this offset,
public static final int MY_RESULT_CODE = Activity.RESULT_FIRST_USER + 1;