Relationship properties in 'Oracle SQL Developer Data Modeler'

I have four questions regarding to 'SQL Language' and 'Oracle SQL Developer Data Modeler':

1. What is the meaning of source optional or target optional in 'Oracle SQL Developer Data Modeler' relationships?

I it something related to'Ordinality' of the relationship?

Actually I make two different logical models. In the first one the source is optional, while in the second one the source is NOT optional.

But the Generated DDL files for both of them are them same. Why?

2. Why when we make a 1:N relationship, 'Identifying' in logical models, then the 'target optional' check box becomes DISABLED??

3. Why an M:N relationship can not be 'identifying' e.g. in M:N relationships the identifying check box is disabled?

4. What does 'in Arc' check box means in relation properties window?