Datasheets typically contain basic motor parameters like torque constant, voltage constant, resistance, and inductance which are all measurable parameters. It is not recommended to use any of the calculated parameters on the datasheet (which is anything that is not measurable).
Km is a well known and important figure of merit for a motor, unfortunately suppliers calculate this term differently and their calculation is sometimes not in the correct units to compare to other motors.  Motor rated torque is probably the most important figure of merit, and it is the least accurate and least understood.
There are several other things that are not present like cogging torque, hysteresis torque, torque linearity, eddy current torque, thermal time constant, thermal resistance, and torque versus angle curves (an indication of torque ripple). If you are designing a system with a motor in it, either select a motor that is well above what you need, or talk directly to the supplier to get all of these things that are not in the datasheet.
Motors are easy to get quickly online from shell companies, and in that case you get even less data and your second purchase may not result in the same performance.