The Property Growth / Depreciation Rate preference is the initial, default rate at which property, at least once purchased, will increase or decrease in value. A negative value could be entered in cases where a depreciation rate is deemed appropriate.
The setting in Plan Preferences, to the right side of the Preferences screen, will apply to the client case that you are presently viewing. This assumption can also be edited on a per scenario basis within the client case.
The setting in System Preferences, to the left side of the Preferences screen, is used only as an initial default in newly created client cases. System Preferences serve only as a template set of assumptions. When you create a new client case, a copy of the System Preferences will be written into your new client's Plan Preferences.
For a future property purchase, the Inflation Rate is used to compute the future value of a property when purchased.
This default setting can be overridden for any individual property or asset in the plan by adjusting the Growth / Depreciation Rate % on the Property / Assets screen, in Advanced Settings > Details.
Plan Preferences vs. System Preferences
Changes to this setting on the mirror Default Inflation / Growth Rates panel in System Preferences, on the left side of the screen, will be used only going forward, as you create new client cases. System Preferences are used as a template only for new client cases. Changes to System Preferences will not retroactively affect your existing client cases.
If you want to change this preference in any existing client cases, you will need to open and edit them individually, in each case's Plan Preferences.