From Mike Macgirvin

ostatus-1.0-draft-2-specification.html
Section 10

Attention: When a user posts an update to the attention of another
user or a group, their server sends the update to the salmon endpoint
of that user or group. The update MUST have the ostatus:attention
link set to the URI of the receiving user or group.

It would help to have an example somewhere of the use of this field, and
indeed many of the events in section 10. It is difficult to create a
protocol compatible instance of these items if there is no documentation
as to what the exact protocol looks like. The doco suggests it is a link
element.

Actually it turns out there is an example shown in
http://code.google.com/p/salmon-protocol/issues/detail?id=1

but the conversation surrounding it seems to indicate that the OStatus
form should no longer be used in favour of the emerging salmon
mechanisms (which likewise are poorly documented). But it does indicate
ostatus:attention isn’t a link element but a tag. This should be spelled
out in the protocol definition, not buried in the administrative
documentation for another protocol. But some clarification should also
be provided as to whether this is affected by the outcome of the salmon
changes and which mechanism should take precedence.