Open-core model

The open-core model is a business model for the monetization of commercially produced open-source software. Coined by Andrew Lampitt in 2008, the open-core model primarily involves offering a "core" or feature-limited version of a software product as free and open-source software, while offering "commercial" versions or add-ons as proprietary software.
The concept of open-core software has proven to be controversial, as many developers do not consider the business model to be true open-source software. Despite this, open-core models are used by many open-source software companies.
Use of contributor license agreements.
Some open-core products require their contributors to sign a contributor license agreement, which either dictates that the copyright of all contributions to the product become the property of its owner, or that the product's owner is given an unlimited, non-exclusive license to use the contributions, but the authors retain copyright ownership. In an open-core scenario, these agreements are typically meant to allow the commercial owner of the product (which in some cases, is ultimately the copyright holder to all of its code, regardless of its original author) to simultaneously market versions of the product under open-source and non-free licenses. This is in contrast with more traditional uses of CLAs, which are meant solely to allow the steward of an open-source project to defend its copyright, or guarantee that the code will only ever be made available under open-source terms, thus protecting it from becoming open core.
Examples.
Restrictions on use in services.
A new variation of the practice emerged in 2018 among several open core products intended for server-side use, seeking to control use of the product as part of a service offered to a customer. These practices, in particular, target incorporation of the software into proprietary services by cloud application service providers such as Amazon Web Services, but with what vendors perceive to be inadequate compensation or contributions back to the upstream software in return.
MongoDB changed its license from the GNU Affero General Public License (a variation of the GPL which requires that the software's source code be offered to those who use it as a service over a network) to a modified version of the GNU General Public License version 3 titled the "Server Side Public License" (SSPL), where the source code of the entire service must be released under the SSPL if it incorporates an SSPL-licensed component. Bruce Perens, co-author of The Open Source Definition, argued that the SSPL violated its requirement for an open license to not place restrictions on software distributed alongside the licensed software. Several major Linux distributions dropped MongoDB after the change, considering the new license to be discriminatory against commercial use.
Redis Labs made its Redis plugins subject to the "Commons Clause", a restriction on sale of the software on top of the existing Apache License terms. After criticism, this was changed in 2019 to the "Redis Source Available License", a non-free license which forbids sale of the software as part of "a database, a caching engine, a stream processing engine, a search engine, an indexing engine or an ML/DL/AI serving engine". The last versions of the modules licensed solely under the Apache License were forked and are maintained by community members under the GoodFORM project.