A voluntary or variable contribution in Arbor means that parents can choose what to pay.
What is the feature called?
This feature is called Variable contribution. Ticking this box when setting up your club allows parents to pay any amount (including not paying at all), rather than a set price.
This feature is called Voluntary Contribution. When setting up a trip, for each price you select whether to enable this to allow parents to choose the amount to pay.
Can they pay more?
Parents can pay more than the amount specified and provide additional funds to support the school.
If you wanted to enable them to pay more, you'd need to have voluntary or variable contributions set to be off, and set up a donation: Donations or voluntary payments
Can they pay less or nothing?
Parents to pay less than the specified amount, and Arbor will automatically create an invoice, and a credit note for the remaining balance. In this case, the invoice will be marked as paid, and parents will not be able to make further payments.
For Trips, parents can select the option to sign up without paying. In this case, no credit note or invoice is created.
There isn't a way to stop parents from paying less than the specified amount without also disabling the ability for them to pay more than the specified amount.