In which category is the supercomputer SUMMIT according to Flynn's taxonomy?
Summit is a parallel system that apparently contains many nodes, where each node is powered by one or more POWER9 processors.  Each POWER9 processor contains a vector scalar unit that Wikipedia says can be described as a SIMD unit.  However, the Summit system as a whole would normally be used in a MIMD mode, as each POWER9 processor can run a separate program. It doesn't have to -- you could run the same code on each node, or different code on each node -- but each node runs asynchronously and independently, so I would classify the entire system as MIMD.