How does section 1170(b) of the California Criminal Code apply to sentencing discretion?

the fact of any enhancement upon which sentence is imposed under any provision of law." ( 1170(b).) A trial court has broad discretion to impose sentence under section 1170 and abuses that discretion when "it relies upon circumstances that are not relevant to the decision or that otherwise constitute an improper basis for decision." (People v. Sandoval (2007) 41 Cal.4th 825, 847.) However, "[a] single factor in aggravation is a sufficient basis for a sentencing choice. [citation]." (People v. Castellano (1983) 140 Cal.App.3d 608, 615.)