słownik angielsko - angielski
- it is possible if you want it but you do not have to have it or do it po angielsku:
1. 
optional
The word "the" is optional in this sentence.
Although it was optional, everyone wore their formal clothes.
is english your optional subject or obligatory?
In some countries, tipping a waiter isn’t optional, it’s obligatory.
He thinks Phisical Education should be optional.
An air conditioner is available as an optional extra.