How to use adb forward

If you’ve tried to use adb forward, you might have noticed that the official documentation is missing a lot of information. adb --help is a lot better:

forward [--no-rebind] LOCAL REMOTE
forward socket connection using:
tcp:<port> (<local> may be "tcp:0" to pick any open port)
localabstract:<unix domain socket name>
localreserved:<unix domain socket name>
localfilesystem:<unix domain socket name>
dev:<character device name>
jdwp:<process pid> (remote only)

If you want even more information, you can check out the source code, specifically SERVICES.TXT. It tells you about the forward command

<host-prefix>:forward:<local>;<remote>
Asks the ADB server to forward local connections from <local>
to the <remote> address on a given device.
There, <host-prefix> can be one of the
host-serial/host-usb/host-local/host prefixes as described previously
and indicates which device/emulator to target.
the format of <local> is one of: tcp:<port> -> TCP connection on localhost:<port>
local:<path> -> Unix local domain socket on <path>
the format of <remote> is one of: tcp:<port> -> TCP localhost:<port> on device
local:<path> -> Unix local domain socket on device
jdwp:<pid> -> JDWP thread on VM process <pid>
or even any one of the local services described below.

and gives more detailed information about what you can put for LOCAL and REMOTE.

dev:<path>
Opens a device file and connects the client directly to it for
read/write purposes. Useful for debugging, but may require special
privileges and thus may not run on all devices. <path> is a full
path from the root of the filesystem.
tcp:<port>
Tries to connect to tcp port <port> on localhost.
tcp:<port>:<server-name>
Tries to connect to tcp port <port> on machine <server-name> from
the device. This can be useful to debug some networking/proxy
issues that can only be revealed on the device itself.
local:<path>
Tries to connect to a Unix domain socket <path> on the device
localreserved:<path>
localabstract:<path>
localfilesystem:<path>
Variants of local:<path> that are used to access other Android
socket namespaces.