Note that the word ‘type’ refers to the symmetry relation. For example, orbital as shown in (i) does not change its sign when it is rotated along the internuclear axis (horizontal axis passing through both the nuclei in this picture). Similarly, (ii) and (iii) also doesn’t change the sign. Hence, there are orbitals are exhibit similar symmetry features as that of a pure sigma bonding molecular orbital. Hence, these are termed as sigma-type orbitals