Can you explain what "spec'd on the plans" means?

When an architect is drafting building plans for a certain number of homes, whether they are single-family homes, multi-family homes, or townhomes, they include all these homes in the project's phase. For instance, they might have a thousand acres to build a thousand or 1,500 homes. The architect will create plans for these single-family homes to develop the neighborhood. They will do all the drawings and specify products for these homes. For example, they might specify using James Hardie board or an equivalent, or BP shingles or an equivalent. The contractors who will build the homes look at these plans and use the specified products.