Clarify meaning of the "updatedafter" request parameter (or drop it)

[kaestli]

It seams that the processing of the updatedafter request parameter is not sufficiently defined. The standards document says only:
"Limit to metadata updated after specified time; updates are data center specific."
I would expect that if I set an updatedafter date, I receive some type of partial inventory response, including modified metadata, but excluding unmodified.

However,

• it is not clear whether the modification date refers to the date when a real modification happened (e.g. when an epoch was closed and a new one opened) or whether it refers to the point in time when this modification was added in the metadata offerings of the service provider.
• it is not clear how partial a response is: If the gain of a channel of a station of the CH network changed, would I receive a stationXML just containing a network element with a station element with a single, modified channel element, or a network element, with a station element containing modified as well as unmodified channels, or would I receive the entire network with all modified and unmodified stations (while not receiving another network, where no station and no channel was modified)
• How is deletion of data handled?

The definition StationXML probably does not suggest obvious answers to these questions, as the format does actually not contain "last modified" dates; the information would need to be derived from other parts of the service providing infrastructure.

As a result I guess, at the time being, the standard does not ensure two fdsn/station service implementations to behave identically.

I would probably recommend to drop this parameter entirely, unless StationXML is extended with modification dates the updatedafter behavioural description can refer to.