ETS catalog is a collector for product data, both ETS Professional and ETS Inside use one.
Product data is the information required by ETS in order to configure a product, i.e the device, it represents. Product data is always offered by the manufacturer of the device. A device can only be added to an ETS project if its product data is present it the ETS catalog.
Online Catalog, is a free of charge service provided by KNX Association, and is hosted on a central server. Manufacturers use this server in order to contribute product data. ETS users use the same server in order to search online for product data within ETS.
For ETS Professional 'Online Catalog' is offered as app, and is free of charge.
For ETS Inside 'Online Catalog' is an integrated feature.