For the purpose of these rules the following principles shall be observed in computing the period of sentence of imprisonment, namely:-
(a) When a prisoner has been sentenced to several terms of imprisonment for several offences, and the sentences of imprisonment have been ordered to run concurrently, then the longest single sentence which the prisoner is undergoing shall be deemed to the term of his imprisonment;
(b) When a prisoner has been sentenced to several terms of imprisonment for several offences and the sentence of imprisonment have been ordered to run consecutively, the total period which the prisoner is to undergo shall be deemed to be the term of his imprisonment;
(c) Remission already earned by the prisoners shall be counted as imprisonment served by him; and
(d) Life sentences shall, for the purposes of these rules, be reckoned as 20 years.
Explanation: The expression "Sentence of imprisonment" in these rules shall include imprisonment in default of payment of fine and imprisonment for failure to furnish security under Chapter VIII of the Code of Criminal Procedure, 1973.