Purpose of a custom hook for a TC hook in libbpf-rs and Linux's traffic control subsystem

Question

I'm seeking to understand the purpose of the "custom" hook here, a snippet of which is reproduced below:

My original interpretation of this code was that it was a way to assign some kind of fixed ID to a BPF program that was loaded, but the custom hook's parent is only set to ingress.

I only need an ingress hook, so I was going to do away with the egress hook - but unsure what to make of the custom hook here

    let mut egress = tc_builder.hook(TC_EGRESS);
    let mut ingress = tc_builder.hook(TC_INGRESS);
    let mut custom = tc_builder.hook(TC_CUSTOM);
    custom.parent(TC_H_CLSACT, TC_H_MIN_INGRESS).handle(2);

    // we can create a TcHook w/o the builder
    let mut destroy_all = libbpf_rs::TcHook::new(progs.handle_tc().as_fd());
    destroy_all
        .ifindex(ifidx)
        .attach_point(TC_EGRESS | TC_INGRESS);

    if opts.query {
        match custom.query() {
            Err(e) => println!("failed to find custom hook: {e}"),
            Ok(prog_id) => println!("found custom hook prog_id: {prog_id}"),
        }
        match egress.query() {
            Err(e) => println!("failed to find custom hook: {e}"),
            Ok(prog_id) => println!("found custom hook prog_id: {prog_id}"),
        }
        match ingress.query() {
            Err(e) => println!("failed to find custom hook: {e}"),
            Ok(prog_id) => println!("found custom hook prog_id: {prog_id}"),
        }
    }

    if opts.detach {
        if let Err(e) = ingress.detach() {
            println!("failed to detach ingress hook {e}");
        }
        if let Err(e) = egress.detach() {
            println!("failed to detach egress hook {e}");
        }
        if let Err(e) = custom.detach() {
            println!("failed to detach custom hook {e}");
        }
    }

    if opts.attach {
        for (i, port) in opts.ports.iter().enumerate() {
            let key = (i as u32).to_ne_bytes();
            let val = port.to_ne_bytes();
            let () = skel
                .maps_mut()
                .ports()
                .update(&key, &val, MapFlags::ANY)
                .context("Example limited to 10 ports")?;
        }
        ingress.create()?;

        if let Err(e) = egress.attach() {
            println!("failed to attach egress hook {e}");
        }

        if let Err(e) = ingress.attach() {
            println!("failed to attach ingress hook {e}");
        }

        if let Err(e) = custom.attach() {
            println!("failed to attach custom hook {e}");
        }
    }

    if opts.destroy {
        if let Err(e) = destroy_all.destroy() {
            println!("failed to destroy all {e}");
        }
    }