Right now, 14.04 is in active development, the detailed Release Schedule of which is hosted here. One can see that the schedule says Alpha 1, Alpha 2 and Beta 1 are for opt-in flavors only.
Can someone please explain the concept of opt-in flavors? What are they and what is different compared to flavors which have opted-out? Who decides upon them and how do I know which flavors have opted-in for a particular development cycle? Do the flavors which have opted-in change between the Alphas and the Betas? And all the points which would help me understand why they are important and have a separate distinction.