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All geometric values in Section C.36.2.2.20.1 are defined in the Beam Modifier Definition Plane.

The Parallel RT Beam Delimiter Boundaries (300A,0649) shall be the positions of the mechanical boundaries (projected on the Beam Modifier Definition Plane defined by the RT Beam Modifier Definition Distance (300A,0688)) between beam delimiter elements. These are fixed for a given beam limiting device. Parallel RT Beam Delimiter Positions (300A,064A) are values specific to a given Control Point, specifying the beam limiting device element openings.

This Section specifies when individual Attributes shall be present in a Sequence.

The RT Control Point Sequence specifies a certain order of execution.

At each RT Control Point the value of various Attributes may be specified as an explicit value (which in the case of a type 2C Attribute may be a null value) and if absent remain at the same value as specified previously. There are physical and mechanical implications of specifying a new value as opposed to staying at the same value, for example gear lash, floating point jitter, etc.

At the first Sequence Item in RT Control Point Sequences (i.e., with an RT Control Point Index (300A,0600) equal to 1) all Attributes affected by this Section shall be present if applicable conditions are met.

For Sequence Items other than the first Sequence Item, Attributes shall be present if applicable conditions are met and the value is different from the previously populated value for the same Attribute (in the case of a type 2C Attribute, a null value is considered as a value). The previously populated value is the value from the Item where the Attribute was present with the greatest value of RT Control Point Index (300A,0600) less than the value of the RT Control Point Index (300A,0600) in the current Item.

This means that for an Item in which an Attribute is absent, the application stays at the value of the previously populated Item.

For Sequences inside a RT Control Point Sequence Item, the Sequence shall be present if any of the nested Attributes affected by this Section differ from the corresponding previously populated Item.

For multi-valued Attributes, such as Parallel RT Beam Delimiter Positions (300A,064A), all values shall be present if any value changes.