Custom Content include any game resources that is not part of the module.
Not all custom content need to be on both the client (players) and server side, most of them only need to be on one side.
For your players to have access to the client side resources, you need put them into hak packs (Haks):
Sinfar has an “Hakset” system which let you make players load different haks base on where they are located in the server for example.
An Hakset is a list of haks, they can be defined in the hakset_list 2da. The HAKSET_TABLE column point to another 2da which list all hak that need be loaded when a player has this Hakset enabled. For example: hakset_default
By default players will load a specific hakset, it is determined in your server config page: