Their main task is assessing about the usability of user interfaces: web pages, desktop/mobile apps, etc.; could be as a member of the development team, or as an independent consultant. They will produce a report with a series of issues and recommendations.
They know the usual heuristics and when/how to use them. Instead of using the heuristics as a checklist to follow it strictly, they usually perform the evaluations based on their own knowledge, detecting the problems and then using the heuristics as a support and reference.
Context of use:
As usability experts, they spend most of the time performing evaluations, recommendations, researching, etc. They work on usability on a daily basis.
Different experts may participate in an evaluation, but usually one of them is who performs most of the work.
As they have a deep knowledge of usability principles, they might use principles and checkpoints from different sets of heuristics for a single evaluation, choosing those more suitable for every particular evaluation.