If you look at the list of FBI directors, some of them are numbered - first, second, third director etc.
But the majority are not numbered and they are marked as Acting.
What is the difference?
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The Director of the FBI can only be appointed subject to the confirmation of the United States Senate. Acting Directors are meant to serve in that position temporarily while the permanent Director is going through this confirmation process.
It is worth noting that no Acting Director was in that post for longer than year, and three quarters of them served for fewer than 100 days.
This is not really specific to the FBI or even to government agencies in general.
Whenever a role X in an organization is currently not filled, but someone temporarily assumes those duties in order to keep the organization functioning, that person is called the Acting X.
It doesn't really matter what the role or the organization is. When the coach of a little league baseball team quits, the person who fills the role until a new coach is found, is the Acting Coach, for example.
In this particular case, the FBI currently does not have a Director, but it cannot function without someone executing those duties. Therefore, a person acts as the Director, and this person is called Acting Director.