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in a vSAN Storage Policy the "Affinity Rule" can only be used with PFTT=0. In this case the VM will be stored on only one site, and you decide on which by setting the Affinity to either the primary or the secondary site.
Only in this case you should make sure that the VM can only run on hosts that are on the site where it is also stored (by using VM/Host DRS rules) to avoid inter-site traffic.
If you use Storage Policies with PFTT=1 for all your VMs then you don't need these rules, because the read locality feature will make sure that data is read locally.