You find this functionality in the view settings and filter toolbar on the right side. You need at least the Project Editor role to add, edit or delete layers.
Click the 
icon in this toolbar to open the Layers form.
Add a Layer
Click the + icon to add a layer to the current area.
Then select and confirm the sheet import on the Import sheet page.
You are then redirected to the Alignment page.
Note: if you want to import a sheet to create a new area, you use the Import Data button for the import.
Edit or Delete a Layer
Click the three-dots menu icon in the Layers form and select Edit to change the name of a layer.
Click the three-dots menu icon in the Layers form and select Delete to delete a layer.
Note that you cannot undo the deletion.
At least one layer must remain in the project, i.e. it is not possible to delete the last layer.
For more detailed information, see our Sphere XG User Manual.