Publishers sometimes offer titles under multiple lending models, usually at different price points. This way, you can choose the model that works best for your needs.
While you're shopping in the One Copy/One User & Metered Access catalog, you'll see that titles available under multiple Metered Access (MA) models or as both Metered Access (MA) and One Copy/One User (OC/OU) will have each lending model option listed in search results and on the title's details page. In most cases, you'll be prompted to choose a lending model before you can add the title to a cart.
Though titles available under the OC/OU or MA lending models can also be available under the Simultaneous Use (SU) or Cost Per Circ (CPC) models, you won't see those lending model options unless you're in their respective catalogs in Marketplace. Similarly, you'll only see the OC/OU or MA options while shopping in the One Copy/One User & Metered Access catalog. You can switch between different catalogs using the Shop drop-down.