This page presents the discussion in the 
IvoaVOEvent working group of how to build a controlled vocabulary for describing:
  
Each of the above is a link; please click to see the whole table. 
The VOEvent community would like your input on these lists, to see if they are sufficient. Here is how you can help.
 
-  Think of an astrophysical process or object that you wish to use in the "Why" (hypothesis) section of a VOEvent
-  Look through the accompanying tables for a description of that event within the controlled vocabulary.
-  Ask yourself if the description is "as good as you can expect in 50 characters", or if there is an essential semantic missing.
-  Report back to the group (voevent at ivoa dot net)