Direction of the U-V Vectors

HFSS uses the U-V vectors to set up a local co-ordinate system. A point on the primary boundary must correspond to that on the secondary. A point on the secondary needs to be paired on the primary so that a one-to-one correspondence can be established. Such a co-ordinate system should be constructed and aligned properly in the same direction so that the mapping is accomplished successfully. Therefore, when the V-vector is directed downwards (when it is below the U vector), you must check the Reverse Direction option to keep it on the face of the box and not pointing outside of it.