In Microsoft SQL Server's linked server configuration, what does the "RPC" setting do?
One allows A=>B the other allows B=>A.  In other words just because server A can call a proc on B doesn't mean B can call a proc on A.

http://msdn.microsoft.com/en-us/library/ms186839.aspx

RPC
   Enables RPC from the specified server.

RPC Out
   Enables RPC to the specified server.