There are three roles that are not assigned.  Rather, they are elected.  Those roles are Secretary, Facilitator, and Rep Link.

Per the Constitution, all core circle members are eligible for election and each may hold multiple elected roles, except for the Lead Link of a circle, who may not be elected as its Facilitator or Rep Link

Elections are held during Governance meetings.  

Each election will be for a fixed term, announced by the Facilitator at the beginning of the process.

Secretary can add an agenda item and select "Hold Election":

The term limit and other information will pop up after that:

For a complete walk-through of the election process see this article: Holacracy Election Process 101: Election Process Walk-through.