By default, Dorico Pro selects enharmonic equivalent key signatures if they have fewer accidentals when transposing selections that include a key signature and in transposing layouts. You can change this setting in each flow independently; for example, if you want all layouts to have flats in their key signatures, regardless of the number of accidentals.
Procedure
Result
Enharmonic equivalent key signatures with fewer accidentals are allowed when the option is activated, and disallowed when the option is deactivated. This affects resulting key signatures when transposing selections that include a key signature and in transposing layouts.