There are two versions of UI5:
- OpenUI5 - Is the open source version, free to use, and released under the Apache 2.0 license. As well as specific SAP UI libraries, OpenUI5 uses many open source libraries. So SAP have chosen to make this version freely available for anyone to use. OpenUI5 can be freely downloaded from Github
- SAPUI5 - Similar to OpenUI5 but includes many more libraries that render the UI for specific SAP features. SAPUI5 is already integrated with SAP HANA Cloud.
The CAP model provides out-of-the-box support for SAP Fiori Elements front ends so developers do not have to write the interface code from scratch.
Watch this video to learn about CDS and i18N Files for UI Development.
Technically, you can switch between OpenUI5 and SAPUI5 as long as you check they both use the same core version.