Earlier, the Software world was rigidly divided between closed and open-source software. Microsoft Windows is closed-source, GNU/Linux is open-source. Microsoft Office and Lotus Notes are closed-source, LibreOffice is open-source. Turbo C++ is closed-source, and GCC is open-source.
But now, a new class of software products has emerged whose core is open-source, but still, the open-source software is of limited use. One model is to offer some critical and useful functionality in a closed-source layer via a managed service in AWS/GCP/Azure, for example, Redis is open-source, but useful modules on top of it are not. Another model is to use licensing gimmickry, for example, MongoDB is licensed under SSPL which requires that if anyone offers MongoDB as a service, then the source code of the full service must be published under this license. The third approach is to make the core software open-source but make it dependent on closed-source cloud services. For example, the node package manager (npm) is open-source, but a closed source company owns the default npm registry. Android is open-source, but most day-to-day application ranging from Google Maps to Google Music are closed-source. Now onwards, rather than calling these softwares open-source, We should call them closeum.