Not all actors are created equal. As such, their relationship to one another is critical in determining a few subtle, yet important, distinctions between actors. In a nutshell, a "lead" is any actor that is not a minion of any other actor (i.e. top-level).
Whenever a new portfolio is created by the user, an initial character is always created as well. This new actor is considered to be a "lead" actor. Similarly, whenever the user creates a new actor in an existing portfolio by means of the "Create New Hero" option on the "Portfolio" menu, the new actor is considered to be a "lead". And the same applies to any new actor that is directly imported from another portfolio, including a stock portfolio.
The exact implications of leads versus non-leads are described in the relevant sections elsewhere in this documentation. However, the distinction is important, so it is set forth here.