The STRING structure defines a string along with the number of characters in the string, as specified in [MS-LSAD] section 2.2.3.1.

 typedef struct _STRING {
   unsigned short Length;
   unsigned short MaximumLength;
   [size_is(MaximumLength), length_is(Length)] 
     char* Buffer;
 } STRING,
  *PSTRING;

Individual member semantics are specified in [MS-LSAD] section 2.2.3.1.