In this step you can make make the relation to source or to target mandatory.

If the 'Order' object type has been selected as the "target" on the 1st page, now you can set whether the relation to 'Order' is mandatory. It means that the 'System' source object type needs to reference at least 1 'Order'.

The target and source 'mandatoriness' options.

This can be easily set later so if you are not sure, you can also skip this step now.

Click 'Next' to proceed to the 4th page.