It's just a heirarchial arrangment for the submissives is all.

Alpha is first then the others,,,and finally omega, some just use numbers like 1st, 2nd, etc: but...unless your going to have a house with a complex hierarchial arrangment for the submissives and the dominants (yes multiple dominants) as opposed to a small harem of one dominanint and maby at best 2-5 submissives where the hierarchy is ussually well established long before a dominant aquires a second girl... there is no real need for it.