Confused between proxy_socket_keepalive & (upstream) keepalive?

[J.R.]

For my setup I use the 'upstream' directive, and in that module there
is the 'keepalive' syntax:

https://nginx.org/en/docs/http/ngx_http_upstream_module.html#keepalive

I just noticed today in the proxy module there is the
'proxy_socket_keepalive' syntax:

http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_socket_keepalive

I'm a little confused... The upstream you set the # of keepalive
connections you want. The proxy module is just an on/off...