INPUT() is a special kind of function expression meant for application elements that are reused in different contexts. The most common example is a data change action that sets the value of a specific column in a row. While the INPUT() function can be used in any expression anywhere in an AppSheet app, it always evaluates to the default value expression unless it is used in the specific scenarios described here (using dynamic INPUTs for data change actions).
INPUT("Age", 20) gives a Number value that defaults to the value 20. This would be used in a data change action (eg: as the value to the set a specific column to). See also: SetColumnValue Actions
INPUT("Feedback", TEXT("")) gives a Text value that defaults to an empty text. This would be used in a data change action (eg: as the value to set a specific column to).
INPUT( inputName, defaultValueExpression )
inputName (Text): Any input name that is unique within the context where it is used.
defaultValueExpression (any type): the default value that should be associated with this expression.
<type determined by defaultValueExpression>: The return type is based on the default value expression. When executed, the INPUT function returns the default value, unless it is in a specific context where the INPUT is supplied (by the app user or by some invoking action).