Certain courses are compulsory, others are optional.

某些课程是必修的，其他是选修的。

《牛津词典》

The course consists of ten core modules and five optional modules.

这门课程包括十个必修单元和五个选修单元。

《牛津词典》

You took an optional course this semester, didn't you?

你这学期选了一门选修课，是吗？

youdao

So, in terms of practicing this weekend, I mentioned that instead of getting a problem-set today, what I am going to be posting is optional extra problems.

而对于这周的练习，我曾经说过今天将不会布置作业，但是会布置一些额外的习题。

麻省理工公开课 - 化学原理课程节选

You have a freewill offering--just because someone wants to do this, a freewill offering. And you have a vow offering that would be offered on the successful completion of a vow, for example. And these sacrifices are all entirely optional.

自由祭品--只因为有人想这么干，自由祭品，还有一个誓言祭品,举个例子，一个誓言如果成功达成，便会供奉祭品，这些祭品都是可选择的。

耶鲁公开课 - 旧约导论课程节选

Seat belts were an optional-use feature in cars starting in 1949, and were made compulsory in the US in 1984.

ENGADGET: Editorial: Countering ID theft requires better awareness campaigns

Medicare's first prescription drug benefit, created late last year for 40 million people, will make it optional--but not mandatory--for seniors to order drugs through the mail.

FORBES: On The Cover/Top Stories

The SIGMA DP3 Merrill is equipped with a hot shoe, allowing use of the dedicated external flashgun EF-140 DG (optional) as well as SIGMA electronic flashguns for SD series such as EF-610 DG Super (optional) and EF-610 DG ST (optional).

ENGADGET: Sigma DP3 Merrill packs a Foveon X3 sensor and a fixed F2.8, 75mm-equivalent lens