The SGML declaration

Defines the concrete SGML syntax used on a system:

• delimiter roles
  what is markup, what is text? e.g. STAGO = <
• reserved names
  what are the strings that mean something to SGML?
  e.g. ELEMENT used in element definitions
• disallowed characters
  which characters are forbidden to appear?
  e.g. control characters (^L, ^G)
• naming rules
  what characters can be used in markup names?
  Case-sensitivity of markup. e.g. it defines that POEM and Poem are the same tag
• quantities
  max. length of documents, markup names,...
  e.g. max. length of GI is 32
• allowed SGML features
  which features are in force?
  e.g.  DATATAG, OMITTAG, FORMAL

The SGML declaration usually used is (something close to) the RCS.

An example: the TEI SGML declaration:

<!SGML "ISO 8879:1986"
-- TEI SGML declaration for local processing with SGMLS:
         extended capacity points
         full ASCII character set instead of ISO 646 subset
         ~Z (26) defined as end-of-file
         Namelen 32, Litlen 512, otherwise as for TEI interchange
   made 16 Aug 1990, rev. 9 Mar 1991 by CMSMcQ 
        increased LITLEN to 2048, GRPCNT to  100 (LB Jul 93) --
CHARSET
BASESET  "ISO 646-1983//CHARSET
          International Reference Version (IRV)//ESC 2/5 4/0"
DESCSET  0       9     UNUSED
         9       1     9
         10      1     10
         11      2     UNUSED
         13      1     13
         14     12     UNUSED
         26      1     UNUSED   -- eof --
         27      5     UNUSED
         32     95     32
        127      1     UNUSED
BASESET  "ISO Registration Number 100//CHARSET
          ECMA-94 Right Part of Latin Alphabet Nr. 1//ESC 2/13 4/1"
DESCSET  128    32     UNUSED
         160    95     32
         255    1      UNUSED
--
CAPACITY PUBLIC "ISO 8879:1986//CAPACITY Reference//EN"
--
CAPACITY  SGMLREF
TOTALCAP    10000000
ENTCAP       500000
ENTCHCAP     500000
ELEMCAP      500000
GRPCAP      1000000
EXGRPCAP     500000
EXNMCAP      500000
ATTCAP       500000
ATTCHCAP     500000
AVGRPCAP     500000
NOTCAP       500000
NOTCHCAP     500000
IDCAP        500000
IDREFCAP     500000
MAPCAP       500000
LKSETCAP     500000
LKNMCAP      500000
 
SCOPE    DOCUMENT
SYNTAX
SHUNCHAR CONTROLS 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
                  16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
                  127 255
BASESET  "ISO 646-1983//CHARSET
          International Reference Version (IRV)//ESC 2/5 4/0"
DESCSET   0     128     0
FUNCTION RE     13
         RS     10
         SPACE  32
         TAB    SEPCHAR 9
     --  EOF    SEPCHAR 26 --
NAMING   LCNMSTRT ""
         UCNMSTRT ""
         LCNMCHAR "-."
         UCNMCHAR "-."
         NAMECASE GENERAL YES
                  ENTITY  NO
DELIM    GENERAL SGMLREF
         SHORTREF SGMLREF
NAMES    SGMLREF
QUANTITY SGMLREF
         ENTLVL   32
         NAMELEN  32
         LITLEN  2048
         GRPCNT  100
         GRPGTCNT  180
         TAGLVL   24
FEATURES
MINIMIZE DATATAG NO     OMITTAG  YES    RANK     NO     SHORTTAG YES
LINK     SIMPLE  NO     IMPLICIT NO     EXPLICIT NO
OTHER    CONCUR  NO     SUBDOC   YES    FORMAL   NO
APPINFO NONE>