what's the difference between proxy_buffer_size and proxy_buffers

[foxgab]

to explain the proxy_buffer_size directive, the document says:

"Sets the size of the buffer used for reading the first part of the response
received from the proxied server"

what does "first part" mean? that is the first packet or what?

what's the relationship and difference between between proxy_buffer_size and
proxy_buffers?
and how these two directives effect the limitation of
proxy_busy_buffers_size?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,275308,275308#msg-275308