When To Use Which Amplitude Unit(s)
Every machine has certain operational characteristics which must be considered when creating the database. Similar machines have similar characteristics and similar (many times identical) database point set-ups. The critical question that must be asked for every machine for which you are creating a database is simply what problems may develop on this machine and what  vibration frequencies will be generated by each of these problems. 

You may need multiple measurements on a 
particular location to get the level of
protection you would like.

In other words, you must create each database point with a specific purpose in mind.
Consider a motor driving some component connected with a coupling. What problems may occur on the motor and what vibration frequencies will each generate ? 
  • Mechanical influences (unbalance, misalignment, etc.) at 1x, 2x, and 3x rpm (also be referred to as orders).
  • Pumps can generate hydraulically-related vibration at the number of vanes x rpm - vane pass frequency.
  • Compressors do likewise at lobe pass and vane pass frequencies (to name only 2 types).
  • Fans can generate at blade pass frequency.
  • With rolling element bearings, vibration at 30kcpm - 50 x rpm (up to 150kcpm) is typically generated during stages leading up to failure.