Under FAR 16.505, every company holding a seat on a multiple-award IDIQ is generally entitled to a fair opportunity to compete for each task order above a set dollar threshold. It does not mean every order gets a full-blown proposal process, fair opportunity competitions are usually faster and lighter than the original award, but it does mean the agency cannot just hand work to a favorite without at least considering the pool. A handful of exceptions let an agency skip it: urgent need, only one holder that can do the work, or a logical follow-on to work already underway.
In a sentence
"This task order will be competed among all IDIQ awardees under fair opportunity procedures." You are guaranteed a real look, not a guaranteed win, so responding fast and well to every order is how vehicle holders actually make their seat pay off.