Let's say you always want to make sure your team adds the extension cables when adding a speaker to an order, but you want the speaker podium to be optional.
When editing the speaker, click into the Relations tab and click 'Accessories'

You'd mark the extension cables as a 'Required' and 'Hidden' Accessory. Marking an item as 'Hidden' will ensure the client doesn't see or be charged the item, but your team will still see the item on their pull sheets. You'd leave those check boxes blank for the speaker podium.

Now when you go to add the Speakers to a project, the 12 Foot Extension cables will be automatically added and the Speaker Podium will be displayed as an optional accessory when you click the "View Relations" button.