This attribute identifies the rest frame in which the source
velocity is stored (the source velocity is accessed using attribute
SourceVel). When setting a new value for the
SourceVel attribute, the source velocity should be supplied in the
rest frame indicated by this attribute. Likewise, when getting the
value of the SourceVel attribute, the velocity will be returned in
this rest frame.
If the value of SourceVRF is changed, the value stored for
SourceVel will be converted from the old to the new rest frame.
The values that can be supplied are the same as for the
StdOfRest attribute (except that SourceVRF cannot be set to "Source"). The default value
is "Helio".
