Delphi Language Guide Delphi for Microsoft Win32 Delphi for the Microsoft. Net framework

String Parameters
When you declare routines that take short-string parameters, you cannot include length specifiers in the parameter
declarations. That is, the declaration
procedure Check(S: string[20]);   // syntax error
causes a compilation error. But
type TString20 = string[20];
procedure Check(S: TString20);
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is valid. The special identifier OpenString can be used to declare routines that take short-string parameters of varying
length:
procedure Check(S: OpenString);
When the 
{$H}
 and 
{$P+}
 compiler directives are both in effect, the reserved word string is equivalent to OpenString
in parameter declarations.
Short strings, OpenString, 
$H
, and 
$P
 are supported for backward compatibility only. In new code, you can avoid
these considerations by using long strings.