React: What is React 19 ref cleanup?

1function TrackedElement() {
2  return (
3    <div
4      ref={(node) => {
5        if (node) {
6          // Setup: observe element
7          const observer = new ResizeObserver(handleResize);
8          observer.observe(node);
9
10          // Cleanup: stop observing
11          return () => observer.disconnect();
12        }
13      }}
14    >
15      Content
16    </div>
17  );
18}

1function DataComponent({ url }) {
2  return (
3    <div
4      ref={(node) => {
5        if (node) {
6          const unsubscribe = subscribe(url, (data) => {
7            node.textContent = data;
8          });
9          return unsubscribe;
10        }
11      }}
12    />
13  );
14}