Determinism is about getting the same outcome in a physical simulation when it is given the same set of inputs. Both Unity Physics and Havok Physics provide deterministic results. However, because different CPU architectures (x86, ARM, etc.) produce minor differences in their mathematical calculations, the deterministic results of the simulation may be different on different hardware.
Because Burst will solve the problem of cross-architecture determinism later this year, Unity Physics will automatically benefit from this feature as well, meaning that the simulation would be the same on any device.
Note: For single-architecture determinism, Unity Physics will require an upcoming Burst update.