The streaming operator uses the terminology pack when you take multiple variables and stream them into a single variable. And conversely, unpack is when you stream a single variable into multiple variables.

Let’s understand through below example:

class packet;

  bit [7:0] s_addr;

  bit [7:0] d_addr;

  bit [7:0] d_length;

  bit [7:0] crc;

  function void pack(ref bit [31:0] bytes[$]);

    bytes = { >> {s_addr, d_addr, d_length, crc}};

  endfunction : pack

  function void unpack(ref bit [31:0] bytes[$], ref packet pkt);

    { >> {pkt.s_addr, pkt.d_addr, pkt.d_length, pkt.crc}} = bytes;

  endfunction : unpack

endclass : packet

program main;

  packet pkt;

  packet pkt_1;

  bit [31:0] bytes[$];

  initial begin

    pkt = new();

    pkt.s_addr   = 8'h8C;

    pkt.d_addr   = 8'h00;

    pkt.d_length = 8'hA4;

    pkt.crc      = 8'hFF;

    void '(pkt.pack (bytes));

    foreach (bytes[i])

    begin

      $display("PACK: bytes[%0d]=%0h", i, bytes[i]);

    end

    pkt_1 = new();

    pkt.unpack(bytes, pkt_1);

    $display("UNPACK : s_addr=%h, d_addr=%h, d_length=%h, crc=%h", pkt_1.s_addr, pkt_1.d_addr, pkt_1.d_length, pkt_1.crc);

  end

endprogram : main