According to the Vedic process, one can get a son through semen (sukra), one can get a bona fide disciple through initiation (savitra), or one can get a son or disciple through the fire of sacrifice (yajna)
SB Canto 9
From Purūravā's rubbing of the araṇis came a fire. By such a fire one can achieve all success in material enjoyment and be purified in seminal birth, initiation and in the performance of sacrifice, which are invoked with the combined letters a-u-m. Thus the fire was considered the son of King Purūravā.
According to the Vedic process, one can get a son through semen (śukra), one can get a bona fide disciple through initiation (sāvitra), or one can get a son or disciple through the fire of sacrifice (yajña). Thus when Mahārāja Purūravā generated the fire by rubbing the araṇis, the fire became his son. Either by semen, by initiation or by yajña one may get a son. The Vedic mantra oṁkāra, or praṇava, consisting of the letters a-u-m, can call each of these three methods into existence. Therefore the words nirmanthanāj jātaḥ indicate that by the rubbing of the araṇis a son was born.