I looked this up recently. It's in fava's help section. The following defines the "number of decimal places" to show in Fava. It's user adjustable.
[Commodities]
Declaring commodities is optional. Use this if you want to attach metadata by currency. If you specify a name for a currency like below, this name will be displayed as a tooltip on hovering over currency names in Fava. Likewise, with the precision metadata, you can specify the number of decimal digits to show in Fava, overriding the precision that is otherwise automatically inferred from the input data.
1998-07-22 commodity AAPL
name: "Apple Computer Inc."
precision: 3