Four alternative but topologically different structures of vorticity exist in rotating 3He−A. As a function of magnetic field ( H) and rotation velocity (Ω), we identify with NMR the type of vortex which is nucleated during cooling from the normal to the superfluid phase. The measurements are compared to the calculated equilibrium phase diagram of vortices in the H−Ω plane at temperatures T≲Tc. Slow transitions are found to reproduce the calculated equilibrium state.