This split determines how any royalties of the resulting work are divided between the producer (who sells the beat) and the buyer.
If the seller declines a publishing split (marked by the red X), then all royalties of the resulting work remain with the producer. On the other extreme, if the seller allows a publishing split of 100% to the buyer, then all royalties of the resulting work belong to the buyer.
Any copies sold below the stated 'sales limit' and any paid streams below the 'paid streams limit' are royalty-free.
The publishing split only applies to copies/streams above those limits.