Rules for the LEN keyword:

Figure 1. Examples of the LEN keyword
 * Use the LEN keyword to define a standalone field of one million
 * characters and a standalone array of 100 characters.
D paragraph       S               A   LEN(1000000) VARYING(4)
D splitPara       S               A   LEN(100) DIM(10000)

 * Use the LEN keyword to define a data structure of length 16000000,
 * and to define three subfields.  Since the lengths of the parameters
 * are less than 9999999, they can be defined using from-and-to, or length
 * notation, or the LEN keyword.
D info            DS                  LEN(16000000)
D   name                          G   LEN(100) OVERLAY(info : 14000001)
D   address                   5000G   OVERLAY(info : 14000301)
D   country               1     40G

 * Use the LEN keyword to define a prototype that returns a varying
 * UCS-2 value that is up to 5000 UCS-2 characters long, and to define
 * two alphanumeric parameters.  Since the lengths of the parameters
 * are less than 9999999, they can be defined either using length notation
 * or the LEN keyword.
D getDftDir       PR              C   VARYING LEN(5000)
D   usrprf                        A   LEN(10) CONST
D   type                         5A   CONST