Digitalisation is a subset of Digital Transformation (DT).
Not a prerequisite, it is simply native to the process (there is no choice on digitalisation, you can’t digitally transform in you’re not digital).
“Thus the first of our Ds is digitalization, for the simple reason that once a process or product transitions from physical to digital, it becomes exponentially empowered.” (Peter H. Diamandis, Steven Kotler, Bold)