The figure element represents some content, optionally with a title and or caption, that is self-contained and is typically referenced as a single unit from the main flow of the document.
During the digital production process for ldsXML there was style information added downstream. During the schemas development the working group has explicitly chosen not to add style information to the content, instead they are relying on semantic based values. This attribute will temporarily store those values. The reason for doing this it keep these values close at hand for a season just in case we need it during the migration period. It is intended to be deprecated from the content and the schema as soon as it is prudent.