Can be anything! In the counterpart it is only being used to reference images generated to the defs section. Inside of defs a series of elements can be predefined and reused by use and thus can reference everything inside this document and possibly outside ?
Parsing use: The properties of use are dominant to the properties of the referenced element, this is not possible with args –args are recessive by default. Done as defined inside the SVG specification.
When the used element isn’t an image a new partial parsing of the referenced element must be started. That is, the referenced element is used as it is.
Every property from this used element has to be inherited to the definition of the referenced element. And then the node itself is to be analyzed.