optional
adj. B1 Intermediate US //ˈɑpʃənəɫ// UK //ˈɒpʃənəl// op·tion·al
adj. available as a choice but not required. You use this to describe something you can decide to do or include if you want to.
adj. available to be chosen but not obligatory. Describes a feature, action, or component that is left to one's discretion.
The extra training session is optional for all staff.
In many programming languages, the final semicolon is optional if the statement is the last one in the block.
While the core modules are mandatory for graduation, students may select from several optional electives to tailor the degree to their specific career goals.
From option + -al.
Usage
Typically functions as a predicative or attributive adjective; often contrasted with 'mandatory' or 'required'.