A pseudovariable represents a target field, for example:
declare A character(10),
B character(30);
substr(A,6,5) = substr(B,20,5);
In this assignment statement, the SUBSTR built-in function extracts a substring of length 5 from the string B, beginning with the twentieth character. The SUBSTR pseudovariable indicates the location, within string A, that is the target. Thus, the last 5 characters of A are replaced by characters 20 through 24 of B. The first 5 characters of A remain unchanged.
Pseudovariables are discussed in Built-in functions, pseudovariables, and subroutines..