The current segment is defined as the segment named in a DELETE, REPLACE, or SELECT option. The current segment has special significance in that it determines what segments can be referenced as variables in an expression.
The current segment name need not be explicitly coded when
Referring to a field in the current segment
Specifying a cast for the current segment
A current segment exists for all expressions except those used in the FIELD keyword of a HEADER, TRAILER, or PREFIX option. In addition, the expression contained in the WHEN keyword of the SAMPLE option can only refer to the root segment. All other expressions can refer to the current segment or any segments in its hierarchic path.