This explanation of Hidden Clients is taken from great explanation done by Napkin with peoro and my help. Thank to them.

  WARNING: This patch needed modification of sv_main.c.
  So in case you decide for the precompiled qvm in this folder,
  make sure you also use the modified binary, else the value of
  sv_maxclients send to clients might be wrong.
  To be on the safe side, we set the default of g_hiddenClients to 0.

  Explanation of hiddenClients:
    HiddenClients are a new class of privileged connections, similar and
    compatible to ordinary privateClients, reserved to admins with
    the # flag. There is no need for a password. Players with this flag will
    automatically be put in this category, if all ordinary public slots are
    used up. Once a public slot gets available, the players will be converted
    to that slot, making it impossible to be used by a public player and also
    freeing up the hidden "slot". Thus, as opposed to privateClients, it tries
    to keep the maximum amount of players near the actual value specified.

  Example:
    - sv_maxclients "20"
    - g_hiddenClients "6"

    This would mean there are (20-6) 14 public slots. Now, if all those slots
    were used up, you and your buddy could still join with flag #. Then there
    would be 14+2 players connected. If 2 other players were to leave, you'd
    be back to 14 (as opposed to 12+2 with privateClients). This means you and
    your buddy took the public slots, thus not letting anyone else take them.
    More than that: Now 6 other players with flag # could connect.

Note: about new tremded, it's fully compatible with other game.qvm (which is compatible with TJW's backported tremded), and game.qvm is fully compatible with TJW's backported tremded. But as said before "value of sv_maxclients send to clients might be wrong"