diff --git a/man/reference-cfg-format.md b/man/reference-cfg-format.md
index 6b55b25ed..81b844b99 100644
--- a/man/reference-cfg-format.md
+++ b/man/reference-cfg-format.md
@@ -89,6 +89,8 @@ To specify the behaviour of functions and how they should be used, `<function>`
 
 The arguments a function takes can be specified by `<arg>` tags. Each of them takes the number of the argument (starting from 1) in the nr attribute, `nr="any"` for arbitrary arguments, or `nr="variadic"` for variadic arguments. Optional arguments can be specified by providing a default value: `default="value"`. The specifications for individual arguments override this setting.
 
+You can specify if an argument is an input or output argument. For example `<arg nr="1" direction="in">`. The allowed directions are `in`, `out` and `inout`.
+
 ### Not bool
 
 Here is an example program with misplaced comparison: