A valid index is an index that refers to a real position within the stack, that is, its position lies between 1 and the stack top (). Usually, functions that can modify the value at an index require valid indices.

Functions that do not need a specific stack position, but only a value in the stack (e.g., query functions), can be called with acceptable indices. An acceptable index can be any valid index, including the pseudo-indices, but it also can be any positive index after the stack top within the space allocated for the stack, that is, indices up to the stack size. (Note that 0 is never an acceptable index.) Except when noted otherwise, functions in the API work with acceptable indices.

For functions that can be called with acceptable indices, any non-valid index is treated as if it contains a value of a virtual type LUA_TNONE, which behaves like a nil value.