Situation requirement codes may be used with a calendar periodicity code in certain circumstances. These situations fall within two general categories:   When the situation governs the scheduling of the requirement   When   the   calendar   periodicity   governs   the scheduling of the requirement For   example,   consider   the   occasion   of   weekly measurement  of  values  when  a  certain  system  is  in operation. The measurement of these values will not be required  when  the  equipment  is  not  being  operated, regardless  of  how  prolonged  the  idle  period  may  be. There   are   cases   in   which   requirements   must   be scheduled  with  regard  to  the  situation  rather  than  the calendar  timing.  The  periodicity  code  will  state  the  R for  situation  first,  and  after  the  hyphen  and  a  unique number, a letter will recognize the calendar contingency.    An    example    of    a    situation-calendar periodicity   code   is   that   an   R-IW   requires   you   to schedule  equipment  lubrication  weekly  when  at  sea. That means that the R-IW is entered into a daily column of  the  weekly  schedule  only  when  the  ship  is  at  sea. During   in-port   times   the   R-IW   will   remain   in   the Outstanding  Repairs  and  PM  Checks  Due  In  Next  4 Weeks column. When the periodicity code is of the calendar-situation  combination,  the  calendar  controls the  scheduling  and  is  only  occasionally  overtaken  by the situation. The calendar periodicity is referred to first in the code, for example, 18M-2R. In the example, the 18M indicates that the longest time between accomplishment   is   every   18   months,   and   the   2R indicates   that   a   situation   could   arise   which   would require it to be done more often. An explanation of such situations will appear on the MRC. When the situation no  longer  exists,  scheduling  reverts  to  the  18-month period. Some examples of the combined calendar and situation requirements are as follows: 7-7 PERIODICITY CODES D - Daily S - Semiannually 2D - Every 2nd day 8M- - Every 8th month 3D - Every 3rd day A - Annually W - Weekly 18M - Every 18 months 2W - Every 2nd week 24M - Every 24 months 3W - Every 3rd week 30M - Every 30 months M - Monthly 36M - Every 36 months 2M - Every 2d month 48M - Every 48 months Q - Quarterly 54M - Every 54 months 4M - Every 4th month 60M - Every 60 months NON-CALENDAR PERIODICITY R - Situation requirement U - Unscheduled maintenance INACTIVE EQUIPMENT MAINTENANCE (IEM) LU - Lay-up PM - Periodic maintenance SU - Start-up OT - Operational test Table 7-1.—Periodicity Codes