“Arm CPU” just means that it uses an instruction set architecture (ISA) licensed from Arm, but the hardware implementation can vary a lot. There is a plethora of Arm-based designs, some from Arm and many from other vendors. Many target the low-power/embedded market, but there are some which have entered the HPC area. Prominent examples are the Fujitsu A64FX and the Marvell ThunderX2. A TX2 system is available in the NHR test and benchmark cluster.