That would depend on what field you work in. For me, working in the chemical engineering field, it was process piping and instrumentation symbols such as valves, pumps, filters, pressure gauges, flexible connectors, hose connectors, etc. When I worked in the civil engineering field it was surveying symbols such as poles, hydrants, catch basins, manholes, trees, traffic signs, fences, mailboxes, water valve boxes, etc.