It lets the child access some content, but not everything. It decides what to block based on labels. A single document can have several labels associated with it, reflecting different perspectives. The parent then chooses which label sources to pay attention to, and which to ignore. Having chosen the label sources, parents also decide which labels indicate material that is appropriate for their children.
Different parents can make different choices.
PICS' contribution is to define a standard format for the labels, so that selection software and labels can be created independently, yet still work together. For example, software from Microsoft is designed to read any PICS-formatted labels, so other organizations can be confident that the software will read their labels, even without making any special arrangements with Microsoft. Companies that prefer to remain values-neutral can provide just the software, not any labels. Values-oriented organizations can provide just the labels, without writing software. PICS is neither the software nor the labels, just a set of technical specifications, the glue that makes the software work with the labels.