ISA-16 Sub-Element / Composite Element Separator: This field provides the delimiter used to separate component data elements within a composite data structure; this value must be different than the data element separator and the segment terminator. In the example above the value is the "greater than" (>), however, it can also be any valid ASCII special character. This is also referred to as the Sub-Element Separator.
Composite Data Elements: Are a set of sub-elements that represent a single named item. For example the REF segment three regular elements and one composite element:
REF01 - Ref ID qualifier (element ID 128)
REF02 - Ref ID (element ID 127)
REF03 - Description (element ID 352)
REF04 - Ref ID (element ID C040)
The regular element separator separates REF01, REF02, REF03 and REF04, and the sub-element separator separates the sub-elements in REF04. If the element separator is "*", the sub-element separator is ">" (according to ISA-16), and segment the Terminator is "~", the REF segment can look like this: