You as an Admin, or user with the relevant permission, can decide what the sequence of a learning paths's courses will be and whether it will be optional or mandatory.
To apply sequence changes to your learning path's course sequence, select the Training Admin workspace. Under Manage Training on the left, select Learning Paths.
Then select the desired learning path from the list provided, then Courses.
A list of all the courses within your learning path will appear. You can change the order of the courses with a simple drag & drop.
By default the sequence is optional and the learner is free to attend the different courses of a curriculum in any order. However, to make the sequence mandatory, just check that option in the Courses Sequence setting.
Please note that if you have added optional courses to your learning path, you will have the ability to skip optional courses and move on to mandatory training.
Apart from defining the sequence of your courses, here you can also use the Curriculum Scheduler to set up a specific number of days after which each course will be available. (Read more on this process here: Using the Learning Path Scheduler