This is the forge version of flan. A completely server side land claiming mod.
For the fabric version go to here
If you wanted to make a public server thats not Bukkit/Spigot etc. you will quickly run up to the problem of not having proper protection of your world since other type of modded servers have as good as no protection mods. With Flan worry no more.
Flan (originally from Fabric LANd) allows you to create and manage claims on your minecraft world. With commands, gui and world interactions you can easily create and manage those. Flan is completly server side only meaning you could setup a fabric server with Flan and connect with a vanilla client and it will work just as fine.
Claims are pieces of land with restricted permissions to prevent griefing with a default depth of 10 (configurable) which means they extend 10 blocks downwards from the claiming spot but no limit upwards. This makes it so it wont affect mining below claims but claims will extend if players start building below. They support subclaims to create town like systems where subclaims have their own permissions too. Being in SubClaim-Mode allows you to edit subclaims (how to down in commands). If you are an admin you can create adminclaims too where only admins (with permission level 2+) can modify and have full access to those.
Head to the wiki for an more indepth explanation at various mechanics and commands.