The secondary axis in the plane, when LinearVelocity/VelocityConstraintMode|VelocityConstraintMode is set to Plane. Value depends on the value of LinearVelocityConstraint/RelativeTo|RelativeTo as follows:

• If LinearVelocityConstraint/RelativeTo|RelativeTo is set to Attachment0, this axis is the Attachment/SecondaryAxis|SecondaryAxis of Constraint/Attachment0|Attachment0.
• If LinearVelocityConstraint/RelativeTo|RelativeTo is set to Attachment1, this axis is the Attachment/SecondaryAxis|SecondaryAxis of Constraint/Attachment1|Attachment1.
• If LinearVelocityConstraint/RelativeTo|RelativeTo is set to World, this value must be specified in the world space.