There is no national nominating process in the United States. Local - County - State - Federal elections all have their own nominating process and qualifications (residency, age, number of supportive voters to qualify, perhaps education)and the successful candidate must satisfy those specific requirements.
During the nominating stage, political parties select their candidates to run for office, while during the election stage, voters choose among these candidates to determine the winner. The nominating stage involves internal party processes, such as primaries or caucuses, while the election stage involves the general electorate casting their votes.
open
the voters
all registered voters can participate
What is it called when voters choose the parties nominees
referendum
All voters from any party are allowed to participate in an open primary. In an open primary, a voter may cast votes on a ballot of any party.
closed
voters participate in elections to choose their representatives in government
a primary election in which voters are not required to declare party affiliation
Mabe because they don't no which to vote for.