Pull requests are a method to facilitate review and management of contribution in distributed software development. Software developers author commits, and present them in a pull request to be inspected by maintainers and reviewers. The success and sustainability of communities depends on ongoing contributions, but rejections decrease motivation of contributors. We carried out a a qualitative study to understand the mechanisms of evaluating PRs in open source software (FOSS) communities from developers and maintainers perspective. We interviewed 30 participants from five different FOSS communities. The data shows that acceptance of contributions depends not only on technical criteria, but also significantly on social and strategic aspects. This paper identifies three PR governance styles found in the studied communities: (1) protective, (2) equitable and (3) lenient. Each one of these styles has its particularities. While the protective style values trustworthiness and reliability of the contributor, the lenient style believes in creating a positive and welcoming environment where contributors are mentored to evolve contributions until they meet the community standards. Despite the differences, these governance styles have a commonality, they all safeguard the quality of the software.