obligatory
Seat-belts are included as standard but electric windows are an optional extra.
My phone came with 20 optional ringtones.我的手机有20种铃声可供选择。
Do come to the party - fancy dress is optional.
There are several optional tours, for which there will be a small charge.
The basic model is quite cheap, but there are a lot of optional extras.

SMART Vocabulary: related words and phrases

Taking and choosing
adopt
adopted
adoption
appoint
bags I...idiom
be your best betidiom
cull
empanel
faute de mieux
favoured
filterable
first choice
picker
Robin Hood
select
selection
selective
selectively
skim
you pays your money and you takes your choice/chanceidiom

optional | American Dictionary

optional
adjective[ not gradable ]
us/ˈɑp·ʃə·nəl/
not necessary or demanded but possible or available; depending on what you decide to do:
I think military service should be completely optional.

optional | Business English

optional
adjective
uk /ˈɒpʃənəl/us
COMMERCE
something that is optional is something that you can buy or do if you want to, but that you do not have to buy or do:
Optional plug-ins cost a further $50 on average.
optional accessories/components
 optional extra
COMMERCE, MARKETING
something that can be added, at extra cost, to a basic product or service to make it more attractive, efficient, etc.:
Some car manufacturers offer advanced stereo systems as an optional extra on their more expensive models.

Examples of optional

optional
We show how to extend any existing parser combinator library with support for parsing permutations of typed, potentially optional elements.
The final (optional) element of the sequence gives the values of the syntactic functions, adjuncts and specifiers (in any order, distinguished by their role names).
Take is usually a transitive verb, yet although it has an obligatory direct-object argument, locative arguments are normally optional.
For these children, then, we have no reason to suspect that agreement was an optional element in the children's grammars.
The only exceptions are found at the word-level, where optional rule application can be systematic in certain grammars.
In section 3.2, we develop an implementation for permutations without optional elements, and in section 3.3 extend this solution to cover optionality.
A postgraduate diploma option can be obtained after completion of the core and optional modules or a master's degree after an additional research project dissertation.
Two further modules have to be chosen from the following optional modules: advanced statistical methods, clinical trials, qualitative research methods, and advanced health economics.
Up until 1945, the optional referendum represented an efficient veto point for groups opposing the welfare state to dilute state-interventionist social policies.
This was accomplished by defining optional matrix columns.
These devices can be combined with optional clausestructure variations, which reflect the fact that word order also encodes discourse status.
Evidently, an interpretation of a derivation is determined by its action on the optional axioms.
The items below are presented as optional ways of writing the same word or expression, some with the apostrophe/stop, others without.
For example, there are positions in which complementizers are obligatory, optional or impossible, and these positions vary across languages and times.
The underlying dynamics of sibling relationships have been identified as balanced reciprocity, optional rather than obligatory exchange of support and the maintenance of personal autonomy.
These examples are from corpora and from sources on the web. Any opinions in the examples do not represent the opinion of the Cambridge Dictionary editors or of Cambridge University Press or its licensors.