A particular object is something more precise than a theme but less precise than a temporal detail.

The themes are generic and a temporal detail is a detail that changes over time (a hairstyle, a coat, a car, a signature etc ...
Postcard themes are generic enough to cover everything but not precise enough if you want to specialize.
For example a card can have flowers as a theme but we do not know what flower it is
or a car whose model or manufacturer cannot be defined.
If we can define a precise model or constructor, we will rather use the temporal detail which makes it possible to locate a map in time.