Many APL functions can be used either with both a left and right argument (for example subtraction), or with only a right argument (for example negation). The functions listed above with a comma or slash in their description are used both ways, with the operation involving two arguments listed first. The remaining functions either require two arguments (for example, comparisons), or have an optional left argument which overrides some default behavior (for example, the format specification). The one exception is Execute, which can only take a single argument.
Operators modify the way that functions behave. The Each operator, for example, applies the function which appears on its left to each item of an array (or pair of arrays) rather than having the function apply to the array as a whole.
Many operators take a single function as a left operand. The Replicate, Expand, and Axis operators, however, take an array as their left operand. And the Inner Product operator takes both a left and a right operand. User-defined operators can take either arrays or functions as operands, and can be defined to take either one or two operands.