Where are Exchange Web Services: ExtendedPropertyDefinition IDs defined?

In researching how to use Exchange Web Services, I see examples like this

Dim PR_DELETED_ON As New ExtendedPropertyDefinition(26255, MapiPropertyType.SystemTime)
Dim PR_SEARCH_KEY As New ExtendedPropertyDefinition(12299, MapiPropertyType.Binary)

That first parameter is an int that represents the property ID. Can anyone give me a pointer to where those ID numbers are defined?

What is Microsoft Exchange ID?

Users can see the unique device identifier called the Exchange Device ID, which is recognized by Exchange. A user can supply this identifier to you so that the device can be approved and allowed access to Exchange services. The Exchange Device ID changes only if the device is restored to factory settings.

Those are called Microsoft Exchange Property Tags and can be found here