In most organizations selection is an ongoing process because

It is true that "in most organizations, selection is an ongoing process."

Let us take this situation as an example.

5 people applied for a specific position in a company. They each pass their resumes and application letters, as well as engage in interviews and exams. At the end of the application selection process, 1 person is successfully hired.

The ongoing process that happens in this example is when the applicants took interviews and exams. From there, one is then "selected" thru the results of interviews and exams.