Source Optional vs Target Optional

Question

Using SDDM 3.3.0.747.
I've got a situation with the Logical Model that is confusing me.  Can anyone shed some light for me?
I have two Entities (i.e. the things that look like tables) in my logical model.  One table is Orders.  The other is Order Detail.
If a row exists in the Order Detail table, it must be tied (via a PK/FK) to a row in the Orders table.  In other words, the Order Detail can't just be a random row -- it has to "belong" to an order.  There can be many order detail rows for a given Order (i.e. you can order multiple things on the same order, and each thing is stored on its own row in the Order Detail table).
However, a single row in the Orders table doesn't necessarily have to be associated with any rows in the Orders Detail table.  For example, perhaps we just started the order and got interrupted before actually adding anything that we wanted to order.  So we can have an order number (PK in the Orders table) that doesn't yet tie to any rows in the Order Detail table.
What I've just described seems to me to be a 1..0M, meaning that a single Order may be associated with any number of Order Detail rows, or none at all.  If the Orders table is on the left and the Order Detail table is on the right, I THINK I should see this connector: -|-----0<-
I have set the Relation Properties as follows:
Source Cardinality
Source: Orders
Source to Target Cardinality: - --<-*  (1 Order for many Order Details)
Source Optional: UNCHECKED
Target Cardinality
Target: Order Detail
Target to Source Cardinality: --1 (1 Order per Order Detail)
Target Optional: CHECKED
Now here's where my brain is getting all wonky: The O indicating an optional constraint is located on the Orders end of the connection line.  -|O-----<-   and to me, that feels backwards.  It feels like that's telling me that "multiple Order Detail lines can be connected to either zero or 1 order", and that's not correct.  An order detail line MUST be connected to an Order.  (Sure wish I could include a screenshot or two).
I feel that the O should be on the Order Detail end of the line, which to me says "one order is associated with any number of detail lines, including zero".
So to me, the position of the O feels wrong.
I can move it into what I think is the "correct" position only by reversing the CHECKED and UNCHECKED status of the Source Optional and Target Optional boxes.  When I do that, the O moves, but the relation properties screen now appears wrong to me.
I know this has to be really basic Data Modeling 101 stuff, but I'm just not getting it.  And I HAVE had my morning Starbucks, so that's not the trouble.
Any help in getting me thinking straight?

Answer 1

AH-HAH!!!  Now I get it.  If we forget Orders and Order Details and instead look at a list of Women and a list of Children, it makes more sense.
There is a one-to-zero-or-many relationship between Women and Children.   I have a list of Women.  For each woman, it is her option to have children or not.  The option rests with the Woman. 
But a child has no such option. If the child exists, it has no option as to whether or not it had a mother.
So the words 'Target Optional' do, in fact, mean 'The Target Is Optional'. If I am looking at one woman, it is indeed optional as to whether or not that woman has children.  Children (target) are not required (i.e. they are optional) for every woman (source).  Therefore, there will be an O on the relationship cardinality line, indicating that the relationship is optional.
What was hard to explain was the positioning of the O on the cardinality line.  The presence of the O simply means that the relationship is optional.  That much is easy.
But I was expecting the O to be positioned on whichever end of the relationship is the optional one (i.e. children are optional, so the O should be positioned on the children's end of the line), and that is not true.  The position of the O indicates which entity the option rests with.  (Which, I contend, is still backwards, but at least now I can explain it.  I don't like it, but I can explain it.)  The woman may, at her option, have one or more children.  That's the way to translate the cardinality line into spoken words when the O is on the woman's (i.e. source) end of the line. 
Philip, thank you for hanging in there with me.  Correct Answer awarded.