Document URL: https://docs.openshift.com/container-platform/4.1/machine_management/creating-infrastructure-machinesets.html#moving-resources-to-infrastructure-machinesets
if the intention of this document was to give you a have to separate infrastructure workload from normal workload, this is only partially achieved.
in fact it tells you how to move infrastructure workload to infrastructure nodes, but it doesn't tell you how to prevent normal workload from landing on infrastructure nodes.
for the masters the strategy is to use taints. this would work also for the infrastructure nodes, except now all the infrastructure operator needs to support the concept of taint (which may be already true).