The 5.9b refers to the difficulty rating scale of a climb. See below
USA (YDS)
The Yosemite Decimal System is the North, Central, and South American rating system. The first number in the YDS designates the class of the climb (always "5" for free climbs), the second number defines the difficulty. Although the YDS is the most popular rating system in North America, other systems exist. Some may have the older NCCS scale (see below), others may have an old YDS that stops at "5.10" no matter how hard that climb is.
NCSS F7 F8 F9 F10 F11 F12 F13 F14 F15 F16
YDS 5.7 5.8 5.9 5.10b 5.10d 5.11b 5.11d 5.12b 5.12d 5.13a
USA (YDS)
The Yosemite Decimal System is the North, Central, and South American rating system. The first number in the YDS designates the class of the climb (always "5" for free climbs), the second number defines the difficulty. Although the YDS is the most popular rating system in North America, other systems exist. Some may have the older NCCS scale (see below), others may have an old YDS that stops at "5.10" no matter how hard that climb is.
NCSS F7 F8 F9 F10 F11 F12 F13 F14 F15 F16
YDS 5.7 5.8 5.9 5.10b 5.10d 5.11b 5.11d 5.12b 5.12d 5.13a