The measurementSequanceNumber is an ever-increasing counter until it reaches 2^16 and wraps around restarting from zero (2^16=65536, thus instead of getting to 65536 it jumps to 0). The only other time the counter resets to zero is when the tag reboots.
When the battery is getting low, the tag will start spontaneously rebooting on its own more and more frequently, until it will get stuck in an infinite reboot loop when the battery is too dead to support even a single measurement cycle.
Here is an example:
See how at the beginning the measurementSequanceNumber gets nicely all the way to around 65535 and then resets. Then around the middle it starts resetting much earlier than that, getting only to a couple of thousand. Towards the end, the counter barely reaches a couple of hundred. At this point I replaced the battery, but the same trend would continue; the counter will reset sooner and sooner, indicating a spontaneous reboot more and more frequently as the battery gets more and more critical.
Thus if you want to create a “battery is low, please replace it” type of alarm, the best trigger would probably be to check if the counter has reset (=the new value is lower than the previously received value) more than once within one day or something, depending on your setup and preference.