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
Click 'Add advanced settings'
Select 'Optional service'
Select if the optional service should be auto-selected or deselected on the proposal.
Depending on what you choose, the service will be selected or de-selected for the client by default:
