A physical, chemical, or structural function.
This is sometimes called “is a” as in “is a piece of furniture made out of wood.
In this case, the specialized object is a piece of furniture.
I think the most accurate way to think about this particular is is a relationship. A relationship between two objects is an is a relationship between two objects.
Sometimes in my life I would look at a set of objects and say, “Is a.” They would be very different, but when you say it like that, you can imagine being able to pick it up and move it by yourself. You’d be able to open the drawer and pick things up without having to ask anyone, to move things around, or to be asked what they are.
Well, that’s exactly what an object is. Sometimes in my life, I’ve had an object that I’ve been using as a tool for someone else. Like a hammer, or a screwdriver, or a spoon. Or maybe a pen. Or a pencil. I had a set of these things, and one of them was my hammer and I’d use it to do things for someone else. And that’s what I’ve been talking about.
I think of objects in an “is a” relationship as those that are specialized to do one thing. They are used for a single purpose and are usually made for a single person. You don’t see a hammer for a carpenter, but you see one for a mechanic. Likewise, you don’t see a pen for writing, but you see one for someone who is a writer.
I think the word “specialized” is actually a misnomer. Objects can be used for multiple things. For example, I could use a pen for writing, a hammer for doing woodwork, and a paintbrush for painting. And even though Ive used a pen before, when I see myself using one, I will feel a strong need for a new one.
The reason why we use objects for multiple things is because we have specialized parts for each type of use. A hammer may be used for woodwork, but it is also used for hammering nails into wood. Like every person, I need to have a hammer, a paintbrush, a paintbrush set, and a paintbrush set set, so I can do a whole bunch of different things with them.
This is also how a car works. The person who owns it also needs to have parts for different functions and a tool for each one. A car is a fully-functioning machine, and you can use your car to do everything from work to a full time job, and you don’t need to have a car just for its own sake.