The use of MarkedOnly and Mark

Hi,
This query may annoy some people but actually i just want to know the use of marked only because this is something i am using for the first time(may be). In the planning worksheet Form i worte the following code at the On Push of one Menu Item.

ReservationEntry.RESET;
ReservationEntry.SETRANGE(ReservationEntry."Source Prod. Order Line","Line No.");
if ReservationEntry.FIND('-') then
  repeat
    if ReservationEntry2.GET(ReservationEntry."Entry No.",true) then
      if ReservationEntry2."Reservation Status" = (ReservationEntry2."Reservation Status"::trac) then
        if ReservationEntry2."Source Ref. No." <> 0 then
          if PlanningWorksheet.GET("Worksheet Template Name","Journal Batch Name",ReservationEntry2."Source Ref. No.") then
            PlanningWorksheet.MARK(true);
  until ReservationEntry.NEXT = 0;
PlanningWorksheet.MARKEDONLY(true);
SETRANGE("Line No.",PlanningWorksheet."Line No.");

PlanningWorksheet = Record Variable of Requisition Line.