As your attached picture, it should be:
A sip proxy can be understood as an "outbound proxy" or an "inbound proxy". Details explanation can be describe as below:
"Pretend you are a SIP User Agent, perhaps a very simple UA that doesn't even do DNS lookups.
A call coming to you is "inbound".
It may hit your home proxy first, and receive treatment such as being proxied to you based on your registered contact. This is an "inbound service", and your home proxy is acting as an "inbound proxy".
Now you want to make a call. Since you're making the call, it is going "out", or can be called "outbound". You send this call to a proxy, which looks up the request URI in DNS and sends the call on towards its destination. This proxy is providing the "outbound service" of DNS resolution, and is acting as an "outbound proxy" for this call.
On any call there may be any number of inbound and outbound proxies and services. Some proxies may even provide both sorts for a single call.
Assume the simple network UA1----P1----P2----UA2
UA1 places a call to UA2."
//refer: https://www.ietf.org/mail-archive/web/sip/current/msg06276.html