The Active Grid is a special component that attempts to answer one key question. What Cells should be loaded?
In order to answer that question, it tracks a Player object and uses that Player's position in combination with Loading Blueprints in order to determine the Active Cells that it would like to be loaded. It then registers with the World as users of those Active Cells. After processing other registered users, including World Regions and other Active Grids, the World decides which World Cells to ultimately load.
You can add this component by selecting a game object and then selecting from the Top Menu Bar:
Component -> Deep Space Labs -> -> SAM -> Primary Components -> Active Grid
or by selecting a game object and using the Add Component menu via its inspector:
Component -> Deep Space Labs -> -> SAM -> Primary Components -> Active Grid
Even though it's called the Active Grid, in truth this component can track and manage multiple grids of cells, which allows it to be used with multiple World Groupings on a single World.
Each grid is known as an Active Grid Grouping, and each Grouping is able to use a unique Loading Blueprint and settings. For instance, if you want to disable the loading/unloading of Cells for a single Grouping, you can do so without affecting the loading/unloading of Cells on other Groupings.
While each Active Grid can only be synced with a single World at a time, there is no limit to the number of Active Grids that can be synced to the same World.
This allows you to track multiple Players at the same time; the World will process all Active Cells needed by the Players and load the highest quality LOD Asset Chunks needed for each Cell.
This functionality can also be used in games that make use of multiple cameras where you need to switch between the cameras at a moments notice. One camera may be focused on the Player, while another is focused on some distant hallway. The second camera would just use a dummy Player object to force loading of Cells that must be shown by it.
It's important to note that Active Grids are not the only way to answer the question of what should be loaded, even if they are the easiest way.
World Regions are a powerful tool for configuring pre-set Cells that can be loaded at runtime using triggers or other logic. In addition, the World offers methods for adding/removing Cells directly which allow you to implement fully customized solutions.