Welsh taxpayers will cover the rent of a hangar leased to Aston Martin should the firm leave the site in a 30-year deal.
The details of the guarantee forged by the Welsh Government have emerged following questions asked by Plaid Cymru leader Adam Price.
But it has not been revealed how much the deal could ultimately cost.
Mr Price said the public have a right to know "whether this is a proper use of public funds".
The Welsh Government did not respond to BBC Wales questions about the deal.
It referred to an earlier statement, saying: "While we strive to be as open as possible we would always seek the permission of any company we deal with before releasing commercially sensitive information to the media."
The Aston Martin plant at St Athan, where the DBX is planned to be made, is forecast to create 750 jobs.
Welsh Government permanent secretary Shan Morgan declined to give further details on Monday when Mr Price asked at Public Accounts Committee about the value of the liability the Welsh Government faced.
The senior civil servant cited commercial confidentiality, and declined to answer whether the Welsh Government had entered into other arrangements of a similar nature. Mr Price later said on Twitter that her answers were "deeply disappointing".
According to details given in response to a question from Adam Price, the economy secretary Ken Skates confirmed the existence of a "guarantee fee arrangement" between Welsh Government and Aston Martin.
"The guarantee relates to the payment of rent over a 30 year term from 2016 for the Superhangar at St Athan," Mr Skates said.
"I.e. should Aston Martin Lagonda cease to operate from St Athan, the Welsh Government would become liable for the rental payment to Legal and General Pensions Ltd."
He said the rental payment would "cease once an alternative building occupier is in place".
By Felicity Evans, BBC Wales political editor
The Welsh Government has been coy about revealing the levels of support for this deal from the beginning.
We only know they've committed £18.8m in grants because the Information Commissioner rejected their arguments about commercial sensitivities and allowed Freedom of Information requests to succeed.
We do not know what the maximum liability for the taxpayer could be if Aston Martin does cease to operate at the site at some point in the next 30 years.
It is a company with a colourful history, but it's now being described as "the world's fastest growing" car brand.
It is promising 750 well paid new jobs. If they come to fruition, does the level of taxpayer support make them value for money?
The short answer is: We do not know.
It has previously emerged under a Freedom of Information Act request by BBC Wales that the firm has been given a total of £18.8m in grants from the Welsh Government.
Mr Skates had declined to give details about its guarantee fee agreement with the car firm when Mr Price asked about it in the Senedd in September.
That was despite the company itself having referred to such an agreement in documents relating to the flotation of the firm on the London Stock Exchange, according to Mr Price.
The Plaid leader told BBC Wales: "The Welsh Government is happy to put out a press release about the benefits but unless we know the full cost, none of us can actually come to a reasonable conclusion as to what the balance is between the costs and the benefits.
"And in a democracy when public money is being used, the public have a right to know so that they can decide, and their elected representatives in a parliament, whether this is a proper use of public funds."