This feature can either be used to let your clients choose which of your services they would like to receive or expose them to additional services you offer.
Additionally, you can set your proposal to include or exclude an optional service by default.
To set a service as optional:
Create a proposal
Add your service
Check the box 'Mark this service as optional'
Select if this service should be active or inactive by default by setting the service to Included or Not Included in the proposal