symbolSortKeyAsExpression

Sorts features in ascending order based on this value. Features with lower sort keys are drawn and placed first. When icon-allow-overlap or text-allow-overlap is false, features with a lower sort key will have priority during placement. When icon-allow-overlap or text-allow-overlap is set to true, features with a higher sort key will overlap over features with a lower sort key.

This is an Expression representation of "symbol-sort-key".