What is the doctrine of immutability of judgment?

In Uy v. Del Castillo,[1] the Court explained the doctrine of immutability of judgment as follows:
Time and again, the Court has repeatedly held that "a decision that has acquired finality becomes immutable and unalterable, and may no longer be modified in any respect, even if the modification is meant to correct erroneous conclusions of fact and law, and whether it be made by the court that rendered it or by the Highest Court of the land. This principle, known as the doctrine of immutability of judgment, has a two-fold purpose, namely: (a) to avoid delay in the administration of justice and thus, procedurally, to make orderly the discharge of judicial business; and (b) to put an end to judicial controversies, at the risk of occasional errors, which is precisely why courts exist. Verily, it fosters the judicious perception that the rights and obligations of every litigant must not hang in suspense for an indefinite period of time. As such, it is not regarded as a mere technicality to be easily brushed aside, but rather, a matter of public policy which must be faithfully complied." However, this doctrine "is not a hard and fast rule as the Court has the power and prerogative to relax the same in order to serve the demands of substantial justice considering: (a) matters of life, liberty, honor, or property; (b) the existence of special or compelling circumstances; (c) the merits of the case; (d) a cause not entirely attributable to the fault or negligence of the party favored by the suspension of the rules; (e) the lack of any showing that the review sought is merely frivolous and dilatory; and (f) that the other party will not be unjustly prejudiced thereby."[2]

[1] 814 Phil. 61 (2017).
[2] Id. at 74-75; citations omitted.