The Newly Discovered / Defined list in the Software Catalog shows software that matches and should be associated to the Software Products shown in the Products list.
Software Products (the right-pane of the catalog) are like containers for Software Components. These link software components, Application Metering data, Packages (if available), and licensing together for easy configuration and management. This is why they are considered "Managed".
There are specific criteria for components to be linked to Products.
Only Software Components that have associated inventory will show up and associate with a Product. This means computers need to have that software associated with them via Inventory.