In cross-examination of Maggie Bourgeau, Duffy's lawyer focused on that blend of travel for both for parliamentary functions with personal business.
Some of the charges Duffy faces relate to taxpayer-funded trips the RCMP allege were for solely personal reasons, including family visits.
"If you travel from point A to point B and there is both a personal aspect to it, but there's also Senate business involved ... you're entitled to reimbursement?" Donald Bayne asked.
"My understanding is that senators can combine Senate business with a personal trip," said Bourgeau, a 17-year-veteran of Senate administration. "The personal portion of the trip will not be reimbursed."
Crown prosecutor Mark Holmes also asked Bourgeau about a meeting she had with Duffy's executive assistant, Melanie Mercer, shortly after Duffy was appointed to the Senate.
Mercer testified earlier this week.
"She wanted to go over the travel policy," Bourgeau said. "We discussed what would be eligible, how to fill out a claim, taxis ... miscellaneous budget."
Earlier in the day, a lawyer representing Senate argued against the release of a two-year-old audit which examined the residency status of senators.
Max Faille argued the document is protected under parliamentary privilege, which gives legislators immunity from the courts to allow them to carry out their work freely.
"The court's role is to uphold this principle that is fundamental to the separation of powers which is itself, a foundational principle of our democracy," Faille said. "That is our purpose here. It is not to favour any party nor undermine transparency."
The residency audit was conducted in early 2013 by a Senate official after questions arose about Duffy and other senators who claimed living expenses for homes in the capital.
Duffy's lawyer and a number of media outlets want the audit to be made public, saying the Senate is being selective about the documents it wants to release.
One of Duffy's lawyers, Peter Doody, continued to hammer away at this argument Friday.
"The question is not whether the release of the document is necessary to the defence because it will be used," Doody said. "The question is whether the core legislative function of the Senate requires that it be kept secret."
Ontario Superior Court Judge Charles Vaillancourt said he would review submissions from both sides.
"I'm not going to give a judgment at this very moment," he said. "Between now and my return in August, I will hopefully have it done and given to counsel in advance."
Court proceedings will continue for another week before the trial pauses until August.
Duffy has pleaded not guilty to 31 charges of fraud, bribery and breach of trust.
Several of those charges relate to his claim that a home in Ottawa was a secondary residence, which allowed him to claim files for nearly $90,000 in housing expenses.
—with files from Jennifer Ditchburn
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