If you want to specify an OEM or custom sensor, then cover these 10 points and you will have covered all necessary aspects:-
1. Geometry - for example, linear or rotary or curvi-linear or 2D or 3D
2. Full-scale - for example, 360 degrees or 600mm
3. Resolution - in other words, the smallest change that must be measured - for example 0,1 degrees or 0,2mm
4. Repeatability - in other words, the stability of the measurement in terms of going back to the same point - for example repatability = +/-0,025mm
5. Linearity - the maximum allowable deviation from a perfectly accurate reading. You might want to think carefully about this since we often find that what is most important for many aplications is actually repeatbaility.
6. Temperature range - for example, -40 + 85Celsius
7. Electrical supply - for example, 5V, 12V or 24V
8. Electrical output - for example RS232, A/B pulses, 0-10V, 4-20mA
9. Any special information such as - "we want to keep power consumption as low as possible" or "it's for submersion in hot sulphuric acid" or "we're using a capacitive device and we have reliability problems"
10. Quantity - for example, "100/year"