a!StartProcess and a!StartProcessLink

Please find all details you are looking for on this documentation page https://docs.appian.com/suite/help/22.1/Ways_to_Start_a_Process_From_a_Process.html#starting-a-process-from-an-interface

It shows a full comparison of all ways to start a process.

In simple words "a!startProcess" is a function and it can only execute inside a saveInto or a Web API on the other hand in "a!startProcessLink" users are shown the start form or first chained attended node and any subsequent chained forms. After submitting the last form, the chain completes, the user returns to the original interface, and that interface reloads. I hope that explanation will give you brief idea.

you can learn more about them from https://docs.appian.com/suite/help/22.1/Ways_to_Start_a_Process_From_a_Process.html

Thanks