There is no description for port parameter of forwarders in documents

Summary

There is no description for port parameter of forwarders in documents

BIND version used

https://ftp.isc.org/isc/bind9/9.12.1/doc/arm/Bv9ARM.ch05.html#options_grammar

What is the current bug behavior?

forwarders [ port integer ] [ dscp integer ] { ( ipv4_address
	    | ipv6_address ) [ port integer ] [ dscp integer ]; ... };

I don't know what is the effect of the first port parameter.

What is the expected correct behavior?

Need to know the effect of the port parameter.