TM 1-1510-218-10 1-2 when there have been extensive changes made to an illustration. 1-11.  AIRCRAFT DESIGNATION SYSTEM. a.   Example C-12C: C –     Basic mission and type symbol (Cargo) 12 –    Design number C –    Series symbol 1-12.  AIRCRAFT EFFECTIVITY DESIGNATORS AND SERIALIZATION. The   aircraft   effectivity   for   content   within   this manual will be designated by the following symbols: C, D,  D1,  D2,  T,  T1,  and  T2.    These  symbols  may  be used individually or in groups. C All C-12C aircraft. D All C-12D aircraft. D1 C-12D   aircraft   with   serial   numbers   prior   to 84-24375. D2 C-12D  aircraft  with  serial  numbers  84-24375 and subsequent. T All C-12T1 and C-12T2 aircraft. T1 C-12T1  aircraft  with  serial  numbers  85-51262 through     85-51268     and     85-51270     through 85-51272 modified with the cockpit and digital engine instrument upgrade. T2 C-12T2  aircraft  with  serial  numbers  86-60084 through   86-60089,   87-70160,   and   87-70161 modified  with  the  cockpit  and  digital  engine instrument upgrade. The    effectivity    symbols    listed    are    used    in conjunction with paragraph titles, text content, performance  charts  and  graphs,  tables,  figure  titles, and   specific   items   on   illustrations   to   show   proper effectivity of the material as applicable.  If the material applies to all models within the manual, no effectivity designators    are    used.        Where    practical,    to    avoid duplication,  descriptive  information  is  written  to  apply to  all  models  and  split  series  effectivities.    Table  2-1 lists  the  most  significant  differences  between  models including the split series effectivities. Aircraft   serial   numbers   are   used   only   where needed  for  clarification.    Paragraphs,  figures,  tables, and  charts  that  apply  to  specific  model(s)  have  the corresponding   model   designator(s)   placed    following the    paragraph    or    figure    title.        Model    and    serial effectivity  designators  are  also  placed  within  tables, charts,    and    text    where    applicable.        No    effectivity designator is used if a paragraph, figure, table, or chart is common to all models. 1-13.  USE   OF   WORDS   SHALL,   SHOULD,    AND MAY. Within  this  technical  manual,  the  word  "shall"  is used to indicate a mandatory requirement.  The word "should"   is   used   to   indicate   a   nonmandatory,   but preferred,    method    of    accomplishment.        The    word "may"  is  used  to  indicate  an  acceptable  method  of accomplishment. 1-14.  PLACARD ITEMS. Placard    items    (switches,    controls,    etc.)    are shown    throughout    this    manual    in    boldface    capital letters. 1-15.  AVIONICS CHAPTERS. This    manual    contains    four    Avionics    chapters entitled as follows. Chapter 3 Avionics –    Avionics common to all C-12C C-12D1/D2, and C-12T1/T2 Aircraft Chapter 3A Avionics    –    C-12C and C-12D1 Aircraft Chapter 3B Avionics    –    C-12D2 Aircraft Chapter 3C Avionics    –    C-12T1/T2 Aircraft 1-16.  PERFORMANCE CHARTS/GRAPHS. This    manual    contains    two   performance    data chapters entitled as follows. Chapter     7     Performance Data   –   C-12C and C-12D1 Aircraft Chapter  7A  Performance Data –   C-12D2   and   C-12T1/T2 Aircraft