The 32-year-old Spaniard made international headlines last week by signing with the former Virgin team.
But she has admitted that, should Timo Glock or Charles Pic be unable to race this season, she will not automatically step into the MR01.
"I am available but I am a test driver, not the reserve driver," she told the AS sports daily.
"Everything is to be decided by the team," added de Villota, who revealed she will travel to all the grands prix this year.
"If there is an incident with them (the race drivers) ... you will have to ask John Booth, the team boss," she answered.
De Villota is also not contracted to practice on Friday mornings, but would not reveal any further details of her new contract.
"I am blessed by sponsors who have got me into F1 under conditions no one would have imagined were possible. I'm proud of that," she said.