The United States court system has a FAQ for this:
What is a senior judge?
The "Rule of 80" is the commonly used shorthand for the age and service requirement for a judge to assume senior status, as set forth in Title 28 of the US. Code, Section 371(c). Beginning at age 65, a judge may retire at his or her current salary or take senior status after performing 15 years of active service as an Article III judge (65+15 = 80). A sliding scale of increasing age and decreasing service results in eligibility for retirement compensation at age 70 with a minimum of 10 years of service (70+10=80). Senior judges, who essentially provide volunteer service to the courts, typically handle about 15 percent of the federal courts' workload annually.
So senior status judges are considered retired but may still hear cases. They continue to get their normal salary whether they are actively hearing cases or inactive (not hearing cases). Their active/inactive status has no effect on their pay. For that matter, neither does senior status (although perhaps they stop receiving raises after taking senior status).
Judge Maryanne Trump Barry, President Donald Trump's sister, is entitled to a federal salary. She is eighty-one and was actively hearing cases for thirty-three years, from October 1983 to February 2017. She was promoted from a District Court to a Circuit (Appeals) Court in 1999. She retired to senior status at age seventy-four and had eleven years of active service. That itself is enough to qualify her.