MSS Blocks are
・ Bank 5 Dedicated to IO
・PLL output OUT2 or OUT3 of CCC_NW block
The clock can be supplied from either
(Select from the MSS PLL reference clock source pull-down menu on the Clocks tab of the PolarFireSoC MSS Configurator.)

If you use a clock input from the Bank 5 Dedicated IO in an MSS block, this clock input is not available to the FPGA Fabric.

When the PLL output OUT2 or OUT3 of the CCC_NW block is supplied to the MSS block, the PLL outputs OUT0 and OUT1 of the CCC_NW block can be used on the Fabric side.