1.17a was released because of compatibility issues with digital versions (D2D, Steam, Impulse, etc.). This caused the version number not to update. If you have a retail version of 1.17, there won't be a difference. For digital users, you can simply apply 1.17a to 1.17. 1.17a will simply be referred to as 1.17. In the game menu, instead of 1.17a, you will simply see 1.17.