Discussion with colleagues showed that it should be described clearer why why number of fractions is per volume in Course Summary but not per volume in a Treatment Phase. 

Proposed Change: Add a paragraph in the usage section of Course Summary and Treatment phases, explaining that a phase describes one series of equivalent fractions and all volumes treated by a phase are involved in all fractions of that phase. Therefore, fraction counting in a Phase is for that Phase on Procedure level, not per volume. It would not differ between volumes affected in the same phase. On the other hand, the Course Summary, which summarizes contributions from potentially multiple phases, provides for each volume separately in how many fractions this volume was treated. Here the number of delivered fractions may differ between volumes, because each volume may have received contributions from different phases or different combinations of phases.