How should icons be set for Appindicators? Can unique icons be used or should window manager icons be used?

On the appindicator dev page, it states:
The icons need to use icon names from themes, direct paths to icon files are not supported. For example, icon names used with gtk_status_icon_new_from_file() won’t work.
It seems from this, that programs should use standard icons. What are the icons that can be used?

From the example code:
app_indicator_set_attention_icon (indicator, "indicator-messages-new");
Where is "indicator-messages-new" defined?