Sparse dimensions lack data values for the majority of member combinations. Dense dimensions have data values for the majority of member combinations. At least one dense dimension is required. Custom attributes cannot be assigned to dense dimensions. Planning designates the Account and Period dimensions as dense, and the remaining dimensions as sparse. To optimize performance for sparse dimensions, Planning searches for and calculates only occupied data values in each dimension combination, reducing calculation time and disk usage. You can modify these settings. See About Reordering Dimensions and Setting Dimension Density and Order.