Per Davis Division Academic Senate Regulation A553, a student may not obtain transfer credit for courses taken at a non-University of California campus in a term during which the student is registered as a UC Davis full-time student. A variance can be obtained only by petitioning the dean of your college well in advance of the desired registration. For more information, see Concurrent Enrollment.
There are a variety of special programs that allow students to take courses at other UCs, at Community or CSUs, in Sacramento, in Washington DC, or abroad. Each program has its own requirements and application process.