In general, a reallocation rule is simply a way to move an indirect cost that can't be allocated in a perspective to a direct cost group that incurred the cost. In most cases, the cost is incurred in the account that actually used the service but this is not always the case. There are a small set of costs that are billed only to the consolidated (ie. Tax and Support). If you are building a reallocation rule for these services, you might want to spread the cost out among all accounts in that consolidated family due to all benefiting from the service. This is when you would enable the Reallocated Across Accounts/Subscriptions and Region Option.
You can read more about how this option works under Step 4: Specify How the Reallocation Occurs here: Cost Reallocation