Union Government
If there isn't a quorum of Supreme Court Judges available to hold a session, the Chief Justice of India (with the President's prior consent, after consultation with the relevant High Court's Chief Justice) may request a qualified High Court Judge to attend as an ad hoc Judge of the Supreme Court for the necessary period, during which they have all the powers and duties of a Supreme Court Judge.
This is a backup mechanism for when the Supreme Court doesn't have enough judges physically available to hold a valid session - a qualified High Court judge can be temporarily brought in to sit as a full Supreme Court judge until the shortage is resolved.
This provision exists to prevent the Supreme Court's work from grinding to a halt purely due to a temporary shortage of available judges, by allowing a qualified High Court judge to step in on a temporary basis.