UPPERBOUND in LDAP Syntaxes is defined within RFC 2252.

A suggested minimum upper bound on the number of characters in value with a string-based syntax, or the number of bytes in a value for all other LDAPSyntaxes, may be indicated by appending this bound count inside of curly braces following the syntax name's OBJECT IDENTIFIER in an Attribute Type Description. This bound is not part of the syntax name itself.

For instance,{64} 
suggests that server implementations should allow a string to be 64 characters long, although they may allow longer strings.

Note that a single character of the Directory String syntax may be encoded in more than one byte since UTF-8 is a variable-length encoding.

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