Value,Description,Reference
source,The System was the source of the event.,[RFC7970]
target,The System was the target of the event.,[RFC7970]
intermediate,The System was an intermediary in the event.,[RFC7970]
sensor,The System was a sensor monitoring the event.,[RFC7970]
infrastructure,The System was an infrastructure node of the IODEF document exchange.,[RFC7970]
ext-value,"A value used to indicate that this attribute is
        extended and the actual value is provided using the
        corresponding ext-* attribute.  See Section 5.1.1 of [RFC7970].",[RFC7970]