I use an anatomy software called Brain Pro from 3d4medical and it has some flaws because it uses older 2D technology for images and highlights anatomical areas by pins. Can someone clarify for me the difference between the posterior commissure and the tectum of the midbrain? thanks

The tectum refers to the "Roof" of the midbrain, an anatomical area that consists of the superior and inferior colliculi. If you slice the midbrain transversely it is the area dorsal to the cerebral aqueduct.

 

A commissure refers to fibres that connects the L and R brain in some way. For example, corpus callosum, anterior commissure, posterior commissure. Posterior commissure specifically refers to the fibres that join the L and R pretectal nuclei, mediating the consensual pupillary light reflex. Its fibres cross the dorsal aspect of the rostral part the cerebral aqueduct, so its fibres run in the area of the tectum. The nuclei that the posterior commissure connect lie in the tegmentum however, in front of the oculomotor nuclei.