What is RPI calls (user calls from statspack)

I found a couple of references to the statistic "User Calls" in statspack reports as follows:
"Oracle allocates resources to keep track of relevant user call data structures every time you log in, parse, or execute. When determining activity, the ratio of user calls to RPI calls indicates how much internal work gets generated as a result of the type of requests the user is sending to Oracle."

I cannot find a description of RPI.

Does anyone know what it refernces?
Thanks.
Barb

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