If this all sounds like bureaucratic babble, it is. But it's also hugely important to the future fiscal health of Philadelphia and its pension fund, which is only 49 percent funded.
The city Board of Pensions assumes that its $4.5 billion portfolio will grow 7.95 percent each year, in line with many cities' and states' expectations. To its credit, the board has gradually lowered the rate in recent years - it was 9 percent in 2005 - but not fast enough for critics.
"That number should have come down more steeply and more quickly over time," said Sam Katz, chairman of the Pennsylvania Intergovernmental Cooperation Authority, which oversees city finances. "Not too many of us are thinking that we're going to see 8 percent or 9 percent in our retirement accounts in the coming years."
He added that more conservative expectations could safeguard the fund against market volatility like the recent recession, which devastated Philly's already distressed pension fund.
Finance Director Rob Dubow, who chairs the pension board, said that the current rate is in line with the fund's performance in the long run and well below recent returns during the market's postrecession surge. Still, he said, the administration may bring it down further.
"It makes sense to keep looking for ways to reduce it," he said. "We're more conservative than we were before the fiscal crisis."
The Governmental Accounting Standards Board, which sets the official "generally accepted accounting principals reporting standards," said this summer that cash-strapped pension funds will soon have to tie assumptions for part of their plans to the rates of a more conservative investment: 20-year municipal bonds, which right now yield about 4 percent.
Moody's credit ratings bureau also recently encouraged pension funds to use more conservative assumptions.
But it's not an easy task.
Lowering the rate by a single percentage point means the city will have to pay roughly $100 million more into the fund each year - a huge demand on an already squeezed budget.
Katz says that reality may be steering the board, which is made up of Nutter appointees, union representatives and the city controller, away from a more realistic number.
"What you have are people on both sides - the city and the labor side - people who are very conscious of the impact on the city budget," he said. "It isn't just an investment number. It's a budget and political number."
Dubow said the board needs to be sensitive about what sharp decreases in the assumption rate would mean for city services.
"You would be putting in hundreds of millions more a year, and then there's a trade-off on the impact on your budget," he said. "It's not done in isolation."
On Twitter: @SeanWalshDN