The Savings Growth Rate, found in Plan Settings, is used as the default interest rate (Growth Rate) for cash accounts, which are found on the Savings & Investments section of the Dashboard. This default interest rate will be used provided the "Grow by Portfolio / Holdings" option is not selected for the account.
Importantly, this preference sets the initial default interest rate for the software's default cash accounts (e.g. John's Cash). These accounts act as a holding place for ready cash from future lump sum inflows and in some cases, surplus unallocated income that is not scheduled to be deposited to a specific account.
If the "Grow by Portfolio/ Holdings" option is selected, the assumed growth rate for Cash will be taken from the software's market assumptions and used instead.
The default interest rate is taken from Plan Settings, but this default can be easily overridden for any account individually. To change the interest rate for an particular cash account, select the account on the Savings & Investments section of the Dashboard and in the Basics screen, edit the Interest Rate %.