This is a general-purpose property, consisting of a string. This string field can be set to override any parameter of an Arnold core node. This allows you to, for example, access and set Arnold core parameters currently not exposed in the MtoA user interface. The property can be applied to poly-meshes, hair, and lights.
You can query the list of parameters an Arnold core node has by using kick.
For instance, if you want to change a rendering option, you can get the attribute names by running:
kick -info options
For a polymesh node, use:
kick -info polymesh
Always refer to the core Arnold nodes (see bottom of page), and not to the parameter's name as exposed in MtoA.
If you want to set multiple Arnold parameters in a User Options string, you can use any whitespace (spaces, tabs, newlines eg \n) that you would use in an actual ASS file. For example, this would set the enable_procedural_cache and error_color_bad_pixel parameters, and declare and set a user parameter named test:
List of 'kick -info options' available through Kick
List of 'kick -info polymesh' available through Kick