Instruments include those that measure process parameters, such as flows, levels, pressures etc and also those that can actually change the process, such as control valves. These are known as Effectors or Final Control Elements
Instruments are the devices that connect a process control system to the real world of physical equipment, and they are the bedrock of the systems, because you can only control what you can measure and only if you have Effectors.
Some engineering software integrates the production of the P&ID's with the Instrument specifications. Some even claim to be able to automatically configure the Process Automation system, such as popular DCS's. However beware - this is a long way short of completely programming such systems, it really only covers Basic Control
It is not the purpose of the web to cover Instrument Specifications in any detail but they are very relevant to the automation and the automation engineers should have some understanding of them, especially the implications of accuracy and repeatability.