Several months ago Popular Photography tested new Portra 800
and said resolution improved from "step 21 versus 19" in the RIT
Alphanumeric Resolution Target, based on 4x6 prints.  Whazzit?
A picture of the RIT chart is in this PDF file:

http://www.image-engineering.de/de/downloads/Test%20Targets%20O-I-R.pdf

[thanks to Bill C of photo.net for the following]

The description says, "Alphanumeric configuration with frequency
range 1-18 cycles/mm in 25 groups."  With this style of target
(equal width black and white bars) a frequency of 1 cycle per mm
means that one white bar and one black bar together fit in a space
1 mm wide.  So they are just saying that these are the coarsest
bars in their target, and the finest bars can fit 18 pairs of
black/white bars into a 1 mm wide space.

If you take the 18 to 1 scale ratio and raise it to the 1/25 power,
you get about 1.12; ie, each step going 12 percent higher than
the previous.  This is, coincidentally (?) the same rate as the
USAF 1951 res chart, which is typically stated as resolution
doubling every 6 steps, or 2^(1/6)= 1.1225 on your calculator.

Resolving an additional two steps (#19 vs #21) would imply an
improvement of 1.12 x 1.12 = 1.26, or about 26% more resolution.
Roughly 26% anyway (could be anywhere 10% and 40%).