AlternateContent Blocks

An alternate content block allows for an alternative representation of information. In some cases, the desire will be to revise a schema with a newer representation. It will also be common to express visual differences using alternate content blocks. Recall that typically lower-level clients will not have the same capability as their future cousins (i.e., the up level version). As such, any future extension done in the up-level version will need to be expressed in a form that the lower-level version can understand. Hence, the need for alternate representations.