The AntMiner S9 manual is available at https://shop.bitmain.com/files/download/AntMiner%20S9%20Installation%20Guide.pdf
The manual says:
The AntMiner S9 can be set up with three mining pools, with decreasing
priority from the first pool (pool 1) to the third pool (pool 3).
The pools with low priority will only be used if all higher priority
pools are offline.
On older S-series models there used to be a "Pool Balance" setting on the pool screen, below the "pool 1", "pool 2", "pool 3" in your screenshot. From the manual for the old S3:
AntMiner can set up three mine pools, the priority decreases from
first pool (pool 1) to third pool (pool 3). There are three Pool
Failover: Automatic backup when Miner faults;
‘Failover’ is by default. The low priority pool will start to work,
only when the high priority pool encounter fault.
multipool strategy from failover to even share balance.
Change multipool strategy from failover to quota based balance.
I do not know why this feature was removed in later models.