The use of the word "data" suggests that you should first revise serial communication..
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Serial communication sends bits one after another.
After synch has been acheived, at certain times, usually evenly spaced, sometimes on a clock signal edges, the voltage on the line is sampled.
At that time, the values should fall within the voltage ranges specifiied above (for RS422) to be interpreted correctly. That is, the driving circuitry has a responsibility to drive it to a valid value at that sample time. That is, the spec says that a receiver should provide the correct response for any voltage level within the specified range.
I have not checked the RS422 standard for a long time, but I would guess that it is probably device-dependent what happen if the voltage, when sampled, is out of range.
Likely the device receiver would be implemented with a scmidt trigger providing a comparator-like performance but with some hysteresis.
If the standard does not specify the details of the receiver, then the device response would be device-dependent.
Others who have better knowledge of the standard may be able to give you a better answer.