Symbolism, Imagery, Allegory
With the exception of Fortune, most of women in the stories aren't symbols. They're very real, down-to-earth, hugely flawed people, occupying very mundane, literal roles. In the Prologue, Boccaccio even rejects a more symbolic version of womanhood, the Muses, in favor of the earthly women who inspire him on a daily basis.
But it's different for the women of the brigata. Since they're fairly static (they don't change a great deal from start to finish) and iconic (representative of a particular age or personality trait), they kind of beg for literary interpretation.
G.H. McWilliam proposes that the women symbolize the three theological virtues (Faith, Hope, Charity) and the four cardinal virtues (Prudence, Temperance, Justice, Courage). So it falls out like this: Pampinea (Prudence), Fiammetta (Temperance), Filomena (Courage), Emilia (Faith), Elissa (Hope), Neifile (Charity), Lauretta (Justice).
How does this matter in practical terms for The Decameron? It's important to note that it's possible to take allegorical readings too far, so be forewarned. But it's also interesting that the stories on Days 3 (Neifile), 6 (Elissa), and 9 (Emilia) are dominated by the themes of Charity, Hope, and Faith respectively. This interpretation of the female characters, then, can lead to a theory about the larger structure of the work.