Several reviewers asked about definitions for Course, Phase, Session, Fraction. Since these concepts are not consistently used within the RT domain, mCODE should contain definitions to make clear how those terms are used in mCODE.
Course and Phase are somewhat covered in usage description in Course Summary usage, but still would be good to have a dedicated glossary. Here is a draft to start this work in the Codex RTTD Terminology Working Group, partly using language from the usage description:
Course: A Course typically covers one Radiotherapy case for the the treatment of one condition or set of related conditions. In the scope of this IG, the Course is the highest level summary in which treatment is reported. Mostly, a Course includes multiple Sessions, can be divided into multiple Phases, and can last for several months. It is often a clinical decision whether treatments are related sufficiently to be covered in one Course or constitute separate treatment cases handled in separate Courses.
Phase: A Radiotherapy Course can be subdivided into Phases. A Treatment Phase covers the treatment of a set of volumes with given parameters such as modality, which volume or set of volumes are treated, which dose these volume receive and the number of Fractions into which the treatment is subdivided. A Phase therefore represent a series of equivalent Fractions. A Phase may be implemented by a single Treatment Plan or a set of Treatment Plans. In particular, Treatment Plans may be revised during the delivery of a Phase. In the extreme case of adaptive treatment, the Treatment Plan may be adapted each day to deliver the next Fraction of the same Phase. To realize more complex treatment patterns than a single series of equivalent Fraction, multiple Phases can be prescribed and delivered. Often Phases are sequential in time, but they may occur in parallel, overlapping, or in alternating temporal patterns. Some systems record a separate Phase for each target volume. This is possible but not required in this model. Rather a Phase should describe the treatment mode and dosage for all targets that are treated in the same series of Fractions. 
Fraction: Fractions denote the equivalent parts into which the delivery of a Phase or a Treatment Plan is subdivided. 
Session: A Session is one visit of the patient, in FHIR terms an Encounter. In one Session, Fractions of different Treatment Plans may be treated, implementing the same or different Phases depending on the scheduled pattern of Phases. So while each Phase represents a series of equivalent Fractions, not all Sessions with the patient may be equivalent, depending on how the treatment of multiple Phases is scheduled. Fractions of multiple Phases can be treated together in the same Session. In case of medical or technical problems, a Fraction of a Phase may also be delivered over multiple Sessions. If a Fraction cannot be delivered fully in one Session, a so-called partial treatment, it is a clinical decision whether the started Fraction is completed in a subsequent Treatment Session or not. Therefore the actual number of Sessions to deliver a Course of treatment can be higher than the ideal pattern of Phases would require, although this is a special case.
Generally on the level of Phases, the number of delivered fractions is of most interest to know how much of the planned series of Fractions was actually delivered. The Course Summary records in addition how many actual Sessions with the Patient were required for the treatment.