Arkansas is an “open primary” state. You can choose the party’s ballot you wish to vote. This decision does not register you with that party and it is a private decision.
In a primary election if a candidate does not with the majority of the votes (50%+1) there will be a primary runoff election scheduled for a few weeks after the primary election date. The runoff election will have the top two candidates with the most votes to ensure that one of them receives the majority of the voters.
If you have any questions about your state’s primary election, please contact your local election officials.