How does a buck-boost transformer differ from an isolating transformer?

A Buck-Boost transformer is manufactured as an isolating transformer, with separate primary and secondary windings and is shipped from the factory in that configuration. When field connected for a buck-boost application, the primary and secondary windings are wired together which changes the transformer’s electrical characteristics to those of an autotransformer. The primary and secondary windings are no longer isolated as they are connected together.