What is the diffrence between plc and pac as well as scada and hmi?

PLC is catered towards discrete control, where as PAC can process more complicated processes.

SCADA is a system to remotely monitor, control and record readings from field and stores it, where an HMI is used for local operations and testing.

PACs provide a more open architecture and modular design to facilitate communication and interoperability with other devices, networks, and enterprise systems. They can be easily used for communicating, monitoring, and control across various networks and devices employing standard protocols and network technologies such as Ethernet, OPC, and SQL.it  also offer a single platform that operates in multiple domains such as motion, discrete, and process control. Moreover, the modular design of a PAC simplifies system expansion and makes adding and removing sensors and other devices easy, often eliminating the need to disconnect wiring. Their modular design makes it easy to add and effectively monitor and control thousands of I/O points, a task beyond the reach of most PLCs

Typically PLCs have been best suited for machine control, both simple and high speed on the other side, a PAC is geared more toward complex automation system architectures composed of a number of PC-based software applications, including HMI (human machine interface) functions, asset management, historian, advanced process control (APC), and others. A PAC is also generally a better fit for applications with extensive process control requirements, as PACs are better able to handle analog I/O and related control functions. A PAC tends to provide greater flexibility in programming, larger memory capacity, better interoperability, and more features and functions in general.