Question

what is Centre unit cell ?

Answer 1

Body Centred Unit Cells. The unit cell which contains one constituent particle (atom, molecule or ion) at its body centre and other constituent particles are located on the corners is called Body Centred Unit Cells

Answer 2

Each corner of the unit cell is defined by a lattice point at which an atom, ion, or molecule can be found in the crystal. By convention, the edge of a unit cell always connects equivalent points. The face-centered cubic unit cell is the simplest repeating unit in a cubic closest-packed structure.
Hope it helps