Métis and non-status Indians say they are in limbo in terms of education, health and social services because the provinces and Ottawa continue to point to the other as to who is responsible for providing services.
"A casualty of that standoff has been a very large body of people," said Joe Magnet, the lead counsel for the plaintiff, in an interview with CBC News. "This population of 800,000 or so non-status Indians and Métis people have been recorded as the most disadvantaged of all Canadians."
Métis and non-status Indians argue that they should be counted as Indians under the Constitution Act and therefore, it should be the federal government that assumes responsibility for them.
The case began in 1999 when former Congress of Aboriginal Peoples' leader Harry Daniels, who died in 2004, took the federal government to court. After more than a dozen years of legal wrangling, the case finally went to trial in May 2011. It took a Federal Court judge a year and a half to release his ruling that approximately 600,000 Métis and non-status Indians do fall under federal jurisdiction.
That decision meant they could negotiate access to federal programs and services long denied to them.
The federal government appealed.
If the Federal Court of Appeal rules against Ottawa again, it could bolster the plaintiffs' case when it, most likely, ends up at the Supreme Court.
"It could be one of the most significant cases dealing with aboriginal peoples in Canadian history," said University of Ottawa law professor Larry Chartrand in an interview with CBC News. "It has the potential of completely changing the landscape of aboriginal-Canadian relations."
The federal government has estimated it would cost taxpayers $4 billion a year if Ottawa had to assume responsibility.
But Congress of Aboriginal Peoples' National Chief Betty Ann Lavallée says that's just speculation.
"That's just fear-mongering. We don't know at this point. Nobody can project," Lavalée said in an interview. "To me, that's just another way of trying to turn the public's opinion against us."
Magnet adds that, regardless, somebody has to pay for the programs and this case is simply about clarifying which level of government that should be.
So far, the federal government has spent $9 million in legal costs fighting the case.