Example NGINX configuration to do service routing based on headers

nginx.conf

events { worker_connections 2014; }

http {
	include /etc/nginx/conf.d/*.conf;

	default_type text/plain;

	error_log /var/log/nginx/example_debug_error.log debug;
	access_log /var/log/nginx/example_access.log;

	match health_check {
		status 200;
	}

	# map HTTP header Service-Name-Code to a variable
	map $http_service_name_code $service_upstream {
		processPartsOrder	processPartsOrder;
		registerNewAccount	registerNewAccount;
		default			default;
	}

	# map HTTP header Service-Name-Code to a HTTP header named host_dynamic
	map $http_service_name_code $host_dynamic {
		processPartsOrder	domain1.example.com;
		registerNewAccount	domain2.example.com;
		default			default;
	}

	# map HTTP header Service-Version to a variable
	map $http_service_version $service_version {
		"5.4.0.0"	"v5.4.0.0";
		"5.3.9.9"	"v5.3.9.9";
		default		"";
	}

	server {
		listen 8098;
		server_name domain1.example.com;
		status_zone processPartsOrder;
		return 200 "You have reached the processPartsOrder service.\n
Info:
Server Address:\t\t\t $server_addr:$server_port\n
Service-Name-Code Header:\t $http_service_name_code
Service-Version Header:\t\t $http_service_version\n
Host Header:\t\t\t $host
Upstream:\t\t\t $service_upstream
Request URI:\t\t\t $request_uri
Full Upstream URL:\t\t http://$service_upstream$uri$is_args$args\n
Served by NGINX Plus $nginx_version\n";
	}

        server {
		listen 9091;
		server_name domain2.example.com;
		status_zone registerNewAccount;
		return 200 "You have reached the registerNewAccount service.\n
Info:
Server Address:\t\t\t $server_addr:$server_port\n
Service-Name-Code Header:\t $http_service_name_code
Service-Version Header:\t\t $http_service_version\n
Host Header:\t\t\t $host
Upstream:\t\t\t $service_upstream
Request URI:\t\t\t $request_uri
Full Upstream URL:\t\t http://$service_upstream$uri$is_args$args\n
Served by NGINX Plus $nginx_version\n";
	}

	server {
		listen 7001;
		server_name default;
		status_zone default;
		return 200 "Please specify a Service-Name-Code header to reach a service.\n
Info:
Server Address:\t\t\t $server_addr:$server_port\n
Service-Name-Code Header:\t $http_service_name_code
Service-Version Header:\t\t $http_service_version\n
Host Header:\t\t\t $host
Upstream:\t\t\t $service_upstream
Request URI:\t\t\t $request_uri
Full Upstream URL:\t\t http://$service_upstream$uri$is_args$args\n
Served by NGINX Plus $nginx_version\n";
	}

	upstream processPartsOrder {
		zone processPartsOrder 64k;
		server 127.0.0.1:8098;
	}

	upstream registerNewAccount {
		zone registerNewAccount 64k;
		server 127.0.0.1:9091;
	}

	upstream default {
		zone default 64k;
		server 127.0.0.1:7001;
	}

	server {
		listen 80 default;
		location / {
			proxy_pass http://$service_upstream/$service_upstream/$service_version$uri$is_args$args;
			proxy_set_header Host $host_dynamic;
			proxy_set_header X-Forwarded-For $remote_addr;
		}

		location @hc-ppo {
			proxy_pass http://processPartsOrder;
			health_check interval=3s fails=1 passes=2 uri=/ match=health_check;
		}

		location @hc-rno {
			proxy_pass http://registerNewAccount;
			health_check interval=3s fails=1 passes=2 uri=/ match=health_check;
		}

		location @hc-def {
			proxy_pass http://default;
			health_check interval=3s fails=1 passes=2 uri=/ match=health_check;
		}
	}

}

I don't see how the named locations gets selected. Could you explain it?