Using the change documents for event creation is a very useful tool, there are a lot of change documents objects and you can limit them to check specific field changes, but sometimes you do not have a compatible object. you can solve this by creating a new object with a compatible key, the object key needs to be the same as the change document object key which can be seen in SWED. If there isn’t a standard object, for example for classification changes (CD object CLASSIFY) then you can continue from here with the object you have created. If there is a standard object use a function module as en event receiver in transaction SWE2 and raise an event in it (by SAP_WAPI_CREATE_EVENT) for the standard object with the correct key.
A small update (4/2015):
I didn’t notice this when I first wrote this, but there is another (more simple) option, at the end of the table in SWED, you can add a function module, which enables you to change the object type and key to a different object, for example the changes of a purchase order are usually on the CD object EINKBELEG, however changes in a service line of the order are written in the CD object MM_SERVICE. In this case you do have a BOR object (BUS2012) but it is not compatible and with a different key. you can use a function module to link the changes to the desired object/class (I would give an example – but SAP already did – SWE_CD_TEMPLATE_OBKKEY_FB_2 or anyone of the other existing FM in SWED)
Anyhow you can still create an object, although longer it still works:
Error for a standard object
CD object table in SWED
Create a new object with the same key
Event linkage now works for the new object