TIL what is primary asosiated type in Swift

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TIL what is primary asosiated type in Swift. In snippet below in Element because it is in angle brackets <Element> in protocol declaration.

public protocol Collection<Element> : Sequence {

    @available(*, deprecated, message: "all index distances are now of type Int")
    typealias IndexDistance = Int

    associatedtype Element

    /// A type that represents a position in the collection.
    ///
    /// Valid indices consist of the position of every element and a
    /// "past the end" position that's not valid for use as a subscript
    /// argument.
    associatedtype Index : Comparable where Self.Index == Self.Indices.Element, Self.Indices.Element == Self.Indices.Index, Self.Indices.Index == Self.SubSequence.Index


    /// A lot of other stuff...
}