Deciphering the city's “20 percent” surplus

In my 7/25/13 interview with Ben Joravsky, we couldn't bring clarity to a statement that appeared in a 7/23/13 Sun-Times editorial ("TIF cash only a start for CPS"):

The city estimates a surplus this year (half of which goes to the schools) may yield only $10 million for CPS, but it could be larger. It all depends on how carefully the city scrubs each TIF—determining how many dollars are already spoken for and how many aren’t—and what percentage is counted as surplus. The city currently uses 20 percent.

The city uses 20 percent of what, for what?

The answer came in another Sun-Times editorial, this one on 8/4/13 ("A small lifeline for our schools"):

. . . the city historically allows only 20 percent of uncommitted cash to count as surplus. City Budget Director Alex Holt told us last week that 20 percent was "probably the starting point" this year.

In other words: Of each year's budget surplus for which the city has no plans, the city's been able but unwilling to spend four-fifths of it.