Hello,
The modulation(t) is the electrical signal that is carried on the optical carrier (laser). The SF is the symmetry factor that defines the location of modulation on the modulator transfer function. While, SC defines if the modulation occurs on the increasing or decreasing side of the modulation transfer function. You may check Figure 1 of the datasheet of the Dual Port MZ Modulator Measured component for better visualization of these terms, although the parameters are defined differently!
Regards,
Ahmad