SeedDMS is available in two versions, which are both updated frequently.
Version 5.1.x is quite mature and was first released in Feb. 2017. It still receives all updates (including new features) unless they are related to features only available in version 6.0.x. All bug fixes and security fixes will first enter 5.1.x and will than be merged into 6.0.x. If you are running SeedDMS 5.1.x and not missing any features, then there is currently not need to upgrade to 6.0.x.
The latest version of the 5.1.x branch is 5.1.28
Version 6.0.x was first released in Feb. 2020. It has additional features like a scheduler, new workflows and a possibility to restrict operations based on roles. If you are new to SeedDMS or want to setup a new installation, then go for this version.
The latest version of the 6.0.x branch is 6.0.21