DNS Servers and DNS relays can be combined into a pool. If you assign such a pool to a rule, YogaDNS will use a random DNS server from this pool to make a resolution. This will improve the redundancy and distribute the load.
Another scenario for the pools is the use of DNS relays (a.k.a. Anonymized DNS). In this case, a pool should contain relay(s) and DNSCrypt server(s). YogaDNS will forward queries over the relay(s) to the DNSCrypt server(s), and, thus, the queries will be anonymized. If a pool contains more than one server or relay, a random server or relay will be used for each query.
To create a pool, open Configuration->DNS Servers via the menu or the toolbar icon.