In the photo there is a big nut on the pipe going into the controller of this furnace. As piping is connected together coming from the controller and in the opposite direction from the gas line you need a way of connecting both pipes: a union. There is a lot of discussion about there being no “code” about where a union is placed so long as it is accessible. Having said that, in our area the general consensus of most installers is that it should be outside the furnace enclosure. Why? Most of the leaks we find on gas piping are on loose unions. If the union is inside the furnace cabinet and it leaks that gas can be ignited when the furnace is operated. The leaking gas and the open flame burners are not a good combination, a worst case scenario would be a catastrophic explosion. So we, as home inspectors want our clients to be safe and always make a comment about unions in furnaces.