The white area in Spartan is a sheet. If you fill your first sheet, there are 20 others. You select the sheet you want from the Sheet menu. The name of the currently selected sheet is shown in the Spartan title bar.
When you first install Spartan your sheets are pre-named Sheet 1 to Sheet 20. You can rename these from the sheet menu.
In the sheets menu dialog, you can select, rename and reposition your sheets. The letters shown to the left of the sheet names are used to select a sheet using the keyboard only. Shift + the letter selects the sheet without opening the sheet menu dialog.