There are many different CLA's but companies usually use it to get all the rights on the contributed code so that they can for example relicense (also proprietary) as they wish. That's usually bad for a community because it prevent a level playing field where everyone has the same rights and duties. By contrast, with the DCO the developer only guarantees that he has the necessary rights to contribute under the projects license. But no rights are transferred.
They used a contributor license agreement which more or less transfered all the rights from the individual contributor to the company.
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