Using Non-APO External System for Availability Check

Question: Hi APO Gurus,

We'd like to use a non-APO external system to perform
availability check from SAP R/3. Our home-grown system
performs product and location substitution and is a
better fit for our requirements than APO.

Does anyone have experience actually doing this from
R/3? In particular, can you mail or point me to
documentation for any of the following?

USEREXIT_AVAILABILITY_IN
USEREXIT_AVAILABILITY_OUT
ATP00001
EXIT_SAPLATPC_001
EXIT_SAPLATPC_002

As I'm sure you know, SAP documentation on these is
non-existent. Thanks for your help!

Rick

Answer:
I don't know if this is helpful, but I know you can do this.
I know of a company that is doing ATP from R/3 to Adexa.
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