Milestones or Iterations

I am new to GitLab and I am evaluating it for use as an Agile Planning tool. I am an experienced Agile Coach and I am starting my learning with Scrum.

Something that threw me from the start is the difference between Milestones and Iterations. It seems like Milestones have been around for a while and Iterations is a new feature in the product. To spread up my learning cand you please advise the best approach for Scrum. Milestones or Iterations.