Released back in January, Samsung's newest Galaxy A5 model experienced a "critical" security flaw of an undisclosed nature which was found and fixed by a quick patch from Samsung back in early November of this year. Now, it seems another critical flaw is now being patched. This time, the 128 megabyte quick fix does not include any new security patches or other features, though the entire 2016 Galaxy A family is supposed to eventually be getting updated to Android 7.0 (Nougat). Indeed, the new update seems intended entirely to fix up this "critical flaw", though Samsung is being tight-lipped about just what the flaw is, perhaps for security's sake.
As mentioned, the patch weighs in at only 128 megabytes, so it's quite unlikely that Samsung has packed anything else in the update aside from the stated fix for a critical flaw of some sort. The phone is currently running on Android 6.0 (Marshmallow), which carries a number of inherent possible system and security flaws. Samsung could be patching just about anything with Android, TouchWiz, or something deeper in, like the kernel or a security concern within the device's software. The patch is already rolling out to all variants of the 2016 Galaxy A5 worldwide, but it's understandable that Samsung would not want to let out any details on just what the flaw may have been at this stage. Obviously, there could be a few holdouts who either have not taken the patch just yet or who haven't had it reach them yet.
Of course, as with almost any quickie update from a manufacturer, the patch may not have been about security at all. Instead, there could be any number of reasons for the update including blocking an exploit that allowed users to root the device or unlock the bootloader to install custom firmware. The critical flaw could also have been something as simple as a true system flaw that could have caused issues for users down the line. Whatever it may have been, it should be available for all users now, so if you have a 2016 Galaxy A5 variant and can't see it, a reboot may do the trick.