Plans are used to organize, control and monitor deals between you and your trading partner. Multiple plans can be set up by your trading partner, where each plan contains one or more program lines. Each program line must belong to a single plan. Plans can be viewed and signed off in Collaborator.
Multiple settings can be configured by your trading partner. Plans are known as Schemes within your trading partners owner and it is here that they will create the terms for the Plan.
More can be seen on plans here.