The birthday 'something' is an undocumented benefit of being ROP Gold.
Currently, the birthday 'something' has been the purchase of an award ticket at 50% of the usual miles, with a few conditions (first sector in B'Day month +/- 1 month, and some others). The 'something' in 2004/05 was a 1 class upgrade.
There are a few details in older threads...
Cheers, TK