The rate property describes the full interest rate applied to the balance of the account or loan. This is (or should roughly be) the full interest rate used to calculate the accrued_interest. For floating rates the credit spread can be obtained by subtracting the base_rate from the rate.
The rate should be recorded as a percentage in decimal format. ie. 3.5% should be represented as 3.5 and not 0.035. There is no restriction to the precision of the number (number of decimal places).