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