The Annotation Group Number (0040,A180) allows the logical association of multiple annotations within the current SOP Instance. Such linked annotations share an Annotation Group Number, but each annotation is semantically separable. The nature of the association is not defined. The number is not semantically significant.
Note: For instance, the R-wave in several waveform channels may be annotated, and all occurrences of the same R-wave could be linked in an annotation group.