What is the symbol used to represent the input or output field (rectangle or parallelogram)

Not sure what you are looking for. I do not know of such a symbol in Appian.

Symbol used for?

Generally for flow chart, we use parallelogram for the input or output fields.

this is what you want to know right?

In a flowchart a parallelogram symbol is used to represent input and output operations. This shape indicates where data enters or exits a process, such as user input or displaying results.