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An optional component field reads and writes as an option, in a query loop and through a handle.
A host keeps an optional field as two columns, its value and whether it is present, and the compiler read only the first. In a loop e.Follow.target answered whatever number the slot last held, so if let on a cleared field matched and on a set one never produced its value, and assigning some(x) or none put the option object in a typed column, which stored NaN. Through a handle, who.Follow.target answered the bare number. A consumer kept every optional handle as a plain entity and a flag beside it to get round this.
A loop now reads <field>$present beside the value and writes both, and none clears the presence. That column name is part of the entity ABI from this version, beside the view's sparse and dense. A read through a handle or a row is wrapped in $opt, which turns the host's undefined for an absent field into none, and a write in $unopt, which hands it undefined for none. A host's ecs.read has to answer undefined for an absent optional field for this to work; one that answers zero reads every absent field as some(0).
A system's component access follows its calls into other files. Access was inferred from the module's own functions, so a call into another file touched nothing as far as the system was concerned. A system that declared what an imported helper wrote got DS0291 calling the declaration unused, and one that left it undeclared got no DS0288 and __drift metadata that never mentioned the write, so a host built a schedule that refused it when the system ran. Each module now publishes what its functions read and write with their signatures, and the importing module's inference starts from that.