What do question marks mean in function signatures

For example, see signature for [fn:ends-with from XPath and XQuery Functions and Operators 3.1](https://www.w3.org/TR/xpath-functions-31/#func-ends-with):

```
fn:ends-with($arg1 as xs:string?, $arg2 as xs:string?) as xs:boolean

```
```
fn:ends-with(	$arg1	 as xs:string?,
     $arg2	 as xs:string?,
     $collation	 as xs:string) as xs:boolean

```