Wherever there are people, there will be dirt. A custodian/substitute custodian knows this better than anyone. Their job is to keep buildings clean and maintained, especially buildings with lots of people going in and out. They can work for schools, churches, offices, and even museums.
The custodian/substitute custodian spends most of their day cleaning. They need to have the physical stamina to operate cleaning equipment and know how to use various cleaning substances-if they mix the wrong ones, they can accidentally make poisonous gas. They also perform various other tasks, like setting up for events and testing emergency systems.
In their role as a substitute custodian, the custodian/substitute custodian is like a pinch hitter who fills in for any missing colleagues. Basically, the custodian/substitute custodian takes care of building emergencies before they happen and maintain a clean, safe working environment. Most buildings would be lost without them.