Advisors have the option of locking a course on a Student's plan for a specific term as a strong suggestion. This does not force the student to register, but they will not be able to remove the course from the plan or move it to a different term.
Protections occur at the course level, you can't lock a specific section of a course on a plan. Protections are added on the Course Plan tab under the default List view where the advisor approvals occur.
[Version 2.6 circa 2015 April - Advising Module]