Some big law firms do this, They have different tracks for different calibers of candidates.

It begins with the training contract phase, with 3 tracks

Track 1: Bright candidates with excellent academic results and extra-curricular achievements. Marked by the firm for retention to be groomed

Track 2: Better than average candidates with good academic results and extra-curricular achievements. In this track, some will be retained while others will not, depending on their performance in the firm (or whether boss likes them)

Track 3: Average candidates with average academic results. Not marked for retention at all, used as cheap labour and exploited to provide 'support'



And then for those who are retained by the firm:

Track 1: Groomed and marked to be future partners of the firm (I assume that poster was referring to this as OCS)

Track 2: Just being used as 'serfs' and can only promote to Senior Associate at highest after many years (And this should be WOSPEC)