Doubt in the represntation of the PID Controllers in P&ID diagram

OP,
I am not going to recommend literature but instead a visit to your local power plant. The horizontal line represents board or panel mounted. Think of a control room with hundreds of gages mounted on a geometric/process flow driven board or wall so operators can monitor the process. These are physical instruments, dials, gauges, knobs, analog control devices. No horizonal line = local device, so on a P&ID a circle on a line represents an instrument at that or approximate to that point. Next topic will be what happens when you put a box or diamond around the circle and what type of interconnecting lines are shown.