Section 6. If, in a civil action in the supreme judicial or the superior court, or in the land court, to quiet or establish the title to land situated in the commonwealth or to remove a cloud from the title thereto, it is sought to determine the claims or rights of persons unascertained, not in being, unknown or out of the commonwealth, or who cannot be actually served with process and made personally amenable to the judgment of the court, such persons may be made defendants and, if they are unascertained, not in being or unknown, may be described generally, as the heirs or legal representatives of AB, or such persons as shall become heirs, devisees or appointees of CD, a living person, or persons claiming under AB. It shall be unnecessary for the maintenance of such action that the defendants shall have a claim or the possibility of a claim resting upon an instrument the cancellation or surrender of which would afford the relief desired; but it shall be sufficient that they claim or may claim by purchase, descent or otherwise, some right, title, interest or estate in the land which is the subject of the action and that their claim depends upon the construction of a written instrument or cannot be met by the plaintiffs without the production of evidence. Two or more persons claiming to own separate and distinct parcels of land in the same county by titles derived from a common source, or two or more persons having separate and distinct interests in the same parcel, may join as plaintiffs in any action brought under this section.