The set of children of all first and second level nodes begins with a node General and ends with a node Miscellaneous . The second and third levels have numerical designations. These unnumbered subject descriptors are essentially fourth level nodes. Subject descriptors marked by an asterisk have been retired from active usage. Users of the Classification System may still search ACM's online and CD-ROM files using the retired descriptors for items classified before the date the descriptor was retired.
Footnotes indicate the retired dates.
In addition to the subject descriptors printed as a part of the Classification System, proper nouns or implicit subject descriptors can be included under the proper numbered node. For example, Pascal is an implicit subject descriptor under D.3.2 Language Classifications, VM is an implicit subject descriptor under D.4.0 Operating Systems, and Ada Augusta Lovelace is an implicit subject descriptor under K.2 History of Computing. Reviews will be printed in Computing Reviews under the implicit subject descriptors whenever appropriate. General Terms are a defined set of words that typically apply to many areas of the field. The General Terms list is somewhat orthogonal relative to the actual tree.