Optional checks are indicated with a question mark '?' next to them.
At the top of the screen you have two 'remaining checks' counts. The second count is how many optional checks you've left to complete (the first is how many mandatory checks).
Once you have completed the required amount of optional and mandatory checks the 'I'm Done' button will appear. Select this to complete the checklist.
Repeatable checks are indicated with a circular arrow.
When completed once, they'll produce a tick, but remain blue and selectable. This is unlike normal checks, which turn grey and cannot be re-selected after they're complete.
To repeat a check, simply select it again.
It's also possible to have a combination or both repeatable and optional checks in a checklist.
This is indicated by showing a questions mark, within a circular arrow.