Distinguishable components of an implicit group
Each component of an implicit group needs to be recognizable. If either of two different components appears at the same place in a data stream, there must be a distinguishable difference between one component and the other.
Sometimes components of an implicit group may be distinguished because there is something in the data that distinguishes them.
After the first item Line of the Form, the next data object could be another item Line. Or, it could be a Trailer Line. When looking at a particular Line, there is something in the data that identifies that Line either as a Header Line, an item Line, or a Trailer Line. The type Line has been partitioned to distinguish between the different kinds of Lines.