IndexedSeq does not introduce new operations, at least not in its immutable incarnation, but overrides a lots of methods defined in SeqOps to provide more efficient implementations. There are four classes implementing it:

The mutable. IndexedSeq adds the following mutation options: mapInPlace, sortInPlace, sortInPlaceWith, and sortInPlaceBy.

The mutable. ArraySeq is a collection representing an Array. It defines an array method returning an underlying array, and an elemTag method that returns the tag of the element type needed to properly support different kinds of arrays as required by the JVM. Because of this requirement, it has separate implementations for all primitive types including numeric types (in addition to ofByte, there are implementations for all other numeric primitives, not shown on the diagram) and Boolean, AnyRef and Unit.

The immutable. ArraySeq was added in the version 2.13. It is an effectively an immutable sequence that wraps an array and is used to pass varargs parameters. It has the same descendants as its mutable cousin.

Range is an immutable structure that contains integer values. It is defined by start, end, and a step. There are two additional methods available: isInclusive, which is true for Range.Inclusive and false for Range.Exclusive, and by, which creates the new range with a different step but the same start and end.

Vector is an immutable structure that provides constant-time access and updates and fast append and prepend. Because of this, Vector is the default implementation for IndexedSeq, as demonstrated in the following snippet:

scala> IndexedSeq.fill(2)("A")
res6: IndexedSeq[String] = Vector(A, A)

WrappedString is an immutable wrapper over some String. It extends strings with all of the operations defined in IndexedSeqOps.