Yellow Outlines Around Lesson Bubbles (Review Lessons)
You'll see Review Lessons with a yellow outline around them. These Lessons are used to review phrases from previous Lessons within the same Unit.
Whether you've already completed the Review Lesson or not, the circle around the it will start to fill in as you begin working through the previous Lessons. Here's an example:
Since the first Lesson of this Unit has been completed, the Review Lesson (7) has already begun to fill in, to reflect that you have started mastering a portion of the phrases contained inside that Review Lesson.
Note that if you were to jump directly to Lesson 7, without first completing Lessons 1-6 you'd see a warning:
Lastly, note that sometimes you'll see Units with Review Lessons in the middle as well as at the end of the Unit.