The following visual "reveal"(I would say) the meaning and composition of an ISO8583 message type. First bit tells the version of the ISO8583, second defines the class, third is for function to be performed(request/response/etc), and the last one shows origin. See following fig-1.
FIG 1: Message type definitions of each bit.
Here(Mitug) is a resource that helped me a lot to get started, that was referred to me by my friend Syed Kamran. In some places Mitug website doesn't work, atleast a couple of my friends from different locations of the world map reported the website is not opening the page. So for friends who are unable to open the same, rightclick and save as this [PDF] layman's guide to understanding the ISO8583 financial transaction messages.