Если быть более точным, то

В классе %Library.String
в cache 5.0.x:
Parameter TRUNCATE As BOOLEAN = 1;
в 5.1 and later:
Parameter TRUNCATE As BOOLEAN = 0;

Соответственно, смотря какая документация и какая версия cache...

В Caché Upgrade Checklists для версии 5.1:
GCNV.pdf
3.2.6.15 TRUNCATE Default Changed For %Library.String
Strings have, among their other parameters, settings for MAXLEN and TRUNCATE. The
value of MAXLEN specifies the maximum permissible length of the string. The value of
TRUNCATE specifies how to enforce the maximum length limit.
• If TRUNCATE is set to true, Caché will store only the first MAXLEN characters in a
variable declared as type %Library.String ignoring the rest of the string.
• If TRUNCATE is set to false, an attempt to assign more than MAXLEN characters to
the variable will return an error status.
In Caché version 5.1, the default value of TRUNCATE for new instances of %Library.String
will be false. In previous versions it had been true. Note that this applies only to new strings
created in version 5.1. Older items of type string will still have the defaults from the time
they were created."