The transformations in the BIOSTER module are a combination of true bioisosteres and 'bioanalogues', i.e. molecular modifications and analogue design techniques applied in optimisation.
The benefit of BIOSTER is that it introduces a vastly larger set of precedented transformations for the Nova module, complete with references to the primary literature from which they are derived.
For more information about the BIOSTER database, you may be interested in the publication "BIOSTER: A Database of Bioisosteres and Bioanalogues" István Ujváry and Julian Hayward, Methods and Principles in Medicinal Chemistry (Volume 54): Bioisosteres in Medicinal Chemistry (Ed. Nathan Brown) (Wiley-VCH, 2012) [ISBN 978-3-527-33015-7].