Customizer → Designs → click a design → Print Zones tab.

Most zones belong to a product or a category (see Product Print Zones). Use the design level when a zone only makes sense with a particular artwork — a sponsor box that exists in one design's layout, for example.
The storefront shows the union of product, category and design zones for the selected product + design; a zone assigned at several levels appears once.
Two cards:
Should I put my "front chest logo" zone here or on the product? On the product (or its category) — then it works with every design. Only use the design level for design-specific placements.
The zone is assigned but doesn't appear in the storefront. Check the zone is Active in Graphics → Zones and that its view side exists on the model.