Parses the input string for a coordinate system unit value, which defines the coordinate system for other attributes of gradients and patterns .
The allowable values are userSpaceOnUse | objectBoundingBox
If the string is empty, the default value (DefValue) is returned.
If the value is "userSpaceOnUse", the user coordinate system for the element is the coordinate system that results from taking the current user coordinate system in place at the time when the element is referenced (i.e., the user coordinate system for the element referencing this element) and then applying the transform specified for the element.
If the value is "objectBoundingBox", the user coordinate system for the contents of the element is established using the bounding box of the referencing element to which the current element is applied (see Object bounding box units) and then applying the transform specified for the element.
If the value is not specified, then the effect is as if a value of 'userSpaceOnUse' were specified.