Union types with a common tag. And whats special with these tags is that they help in discriminating the types which are combines to make the union type.

For example lets create three shapes circle, rectangle, triangle .

interface Circle {
    type: 'circle';
    radius: number;
}

interface Rectangle {
    type: 'rectangle';
    height: number;
    width: number;
}

interface Triangle {
    type: 'triangle';
    height: number;
    width: number;
}

Now lets create a union type with it

type Shapes = Circle | Rectangle | Triangle

The benefit of having a property which is discriminating from all interfaces is that it helps in narrowing down the type while using it.

Practically, let's define a function which returns area of the Shapes passed in

const area = (shapes: Shapes) {

}

Now if you accessing shapes , it won't provide all properties in each interfaces rather it would provide the property which is common in all interfaces .

Which is actually a good thing because we can use this to narrow down our types . Which essentially means the discriminated property acts as a type guard .So now lets narrow down our types using a switch statement

Since we are discriminating using the common property within the interfaces, now typescript can easily differentiate interfaces within the union type .