There is an excellent article
here from AOII's magazine "To Dragma" about the legacy policy (for AOII but really can be applied to all NPC orgs). It beings on page 4 out of 6 pages.
Basically, yes, a legacy should be given special consideration, but legacies should be qualified in their own right for membership. Therefore, while legacies DO count, there is no 100% certainty guarantee, you know? Also, the collegiate members vote to see who they feel would be best compatible in the chapter, so again, being a legacy does not mean that a rushee is a shoo-in for membership.
Also, another thing too, is that if there are TONNES of legacies going through Recruitment, it is feasible that not all legacies will be extended a bid.
Hybrid Mode
