6.4.4.5.3 Optional
Figure 19 – Example using the Standard ModellingRules Optional and Mandatory

1. There is one more possible combination [A-C-F], see picture.

2. I think the last sentence denote combination A10 not A13 and needs an "e" (... from one F but not from the other, ...).
   Original: In the case of A13, there could be a reference from on F but not from the other, or from both or none of them.