What are the main ways of classifying instruments? What is classification by function?

The image you sent discusses Analogue and Digital Systems and Instrument Classification.

Here's a summary of the key concepts:

• An analogue system processes information as continuous signals, often displayed or recorded directly.
• A digital system converts analogue information into a digital format using an analogue-to-digital converter, which is then processed by a computer or processor.

Instruments can be classified in several ways:

• Classification by function groups instruments that measure the same quantity, such as position, voltage, pressure, or temperature.
• Classification by transducer technology groups instruments that use the same underlying technology or process, such as strain gauge, piezoelectric, or photovoltaic.