Please  consult  SCM  (i.e.,  LC's  Subject  Cataloging  Manual): Shelflisting G60, 3.
Cuttering practices are based on  the Cutter  Table, taking  into  account adjustments to  allow correct filing  and  room  for  later  entries.

The  range .A12-.A19  is  often  reserved for Cuttering for  Roman  or Arabic numerals. ."A" Cutters  may be reserved for other purposes as well, for example, official  publications of corporate bodies (G220).  In  many cases practice will depend upon  the particular schedule, as  well  as existing  shelflist holdings, so questions in  this  regard citing specific  examples can be more adequately addressed.
 
Lenore Bell