RPC vs Document style messaging

[Paul Fin]

Hi,
I have a few doubts regarding RPC and messaging and the
use/need for
Document style and RPC style encoding of SOAP messages. I
have searched on
many websites but I haven't been able to find a convincing
answer.
Is it right to say that when invoking a webservice, a SOAP
message is
serving the purpose of a remote procedure call.

1. I am not able to understand why we require two styles(
Document and RPC
style) for encoding SOAP messages.
2. When is the criteria for using either of the two styles
i.e When should
one use Document style and when the RPC style.