The hierarchy of control is often used as a brainstorming tool to come up with effective controls (aka, barriers). It's good because it favours proactive interventions like eliminating a source of fuel over reactive interventions like putting out a fire. However sometimes it is misused as a formal classification tool.
A barrier can have different states in an incident, mostly divided into four types: 1) Missing barriers 2) Failed barriers 3) Inadequate barriers and 4) Effective barriers. There are considerable differences in interpretation of these states, and when one or the other should be used. Here I'd like to give