where to find SystemTap event definition

Question

where are the SystemTap events defined? For example, the below probe is for vfs.write.return. How do I know I could probe a component called 'vfs', it can be called 'virtualFs' as well. And there is a variable 'devname' defined. Without a document reference, I won't be able to know the name of the component (vfs here) and it's defined variables.

global io_stat,device
global read_bytes,write_bytes

probe vfs.write.return {
  if ($return>0) {
    if (devname!="N/A") { /*skip update cache*/
      io_stat[pid(),execname(),uid(),ppid(),"W"] += $return
      device[pid(),execname(),uid(),ppid(),"W"] = devname
      write_bytes += $return
    }
  }
}