Group Manager is a implementation of permissions. It allows you to set up permissions for groups, and add users to these groups. The result is you can have multiple tiers of access to commands. This is useful for most Minecraft servers, as it lets you build a community, without it you struggle to balance the right number of admins with users.
Group Manager allows you to effectively manage any size server by letting you create as many different groups and subgroups as you could possibly need. This config can be as simple or as complicated as you could wish.
The most traditional system (shown below) uses group inheritance, starting with new players at the top owners on the bottom. On smaller servers you might end up merging the mod and admin rolls, and most people will have different opinions when it comes to which powers are admin powers and which ones mods should get.
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