Please explain what you mean by "unchecked".

As for "mapped", the following may give you what you seek.

For classname, e.g. %Library.String, this expression will return true (1) if the class is mapped from a database that is not the default code database for the current namespace:

##class(%SYS.Namespace).GetGlobalDest(,"^oddDEF",classname)'=##class(%SYS.Namespace).GetGlobalDest(,"^oddDEF")

Settings stored in the XData block of the production class (e.g. values entered through Portal) trump any System Default Settins values.

In Portal there's this button to get you to a page that may help you understand what's going on:

For a long time I've complained that the Portal UI makes it too easy to override a System Default Settings value (which is namespace-specific) with a value that gets stored in the production class (which you might migrate from one namespace to another). Add to this the fact that Portal ignores source control when altering the production class sad

Here's the base implementation of this method in %Library.GlobalIdentifier (2017.1 version):

 /// If a persistent class is GUIDENABLED then this method can be called to override the default GUID assignment.
/// This method accepts a guid value and returns the override value currently assigned. The return value will only
/// differ from the supplied value if the override is unsuccessful. It is only valid to call this method on a new object.
/// The guid value passed to this method is not validated. It is up to the user to make sure the guid is properly formed.
/// The guid assignment does not actually occur until the object is saved. If the object has already been assigned a GUID
/// or if the GUID override value has already been assigned to another object then the GUID override value will be discarded.
/// The check for GUID value uniqueness and a GUID value previously assigned is done only at the time the object is saved and is
/// not performed by this method.
Method %OverrideGuidAssignment(pGUID As %Library.CacheString) As %Library.CacheString
{
set i%"%%GUID" = pGUID
quit i%"%%GUID"
}

If I understand Alex correctly, he's calling this method in order to force a new instance of a GUIDENABLED class to have a specific GUID instead of getting one automatically assigned at save. He's not overriding the class in his own class definition.

I have highlighted in yellow some sentences in the comment that may be relevant.

I have also highlighted in pink the sentence Alex quoted, which is a bit unclear. By "return value" I think it means the GUID of the object instance after the save has subsequently been performed.

Steve, it may be worth you adding the "MultiValue Basic" tag to your question (which you can edit).

I haven't yet seen much MV traffic here on Developer Community. This lookup of posts with the "MultiValue Basic" tag currently only shows ones created by Community Manager. This Google Group has existed for several years, and I wonder if the folk there have yet heard about DC.

Today's DC update means that questions with an accepted answer have the background of their answer count box shown differently. In the screenshot below, the first question has received 3 answers but the OP hasn't marked any of them as accepted. The second has now answers, and the third has two, one of which has been accepted by the OP:

That's good, but when I click into the third question I am no longer shown which of the answers the OP accepted:

 

Prior to the DC update I would have seen a green checkmark alongside the accepted one.

Is this change deliberate, or an unintended side-effect of you no longer showing the checkmarks when they're not clickable (which means they're hidden from everyone except the OP)?