The Chart Supplement is the FAA publication, formerly the Airport/Facility Directory, that lists runway layout, lighting, services, remarks, and frequencies in more detail than the sectional data block. In that supplement, the CTAF is the frequency printed with a C inside a circle. The block on the chart is only the summary.
What the supplement contains
The FAA publishes the Chart Supplement for pilots who need more than a chart symbol. An entry can show every runway and its dimensions, the lighting system, available services, traffic-pattern notes, and special remarks. It is the place you go for the full runway layout and the remarks at an unfamiliar airport. It is not a real-time weather radar picture. It is not your remote pilot certificate number. Weather observations still come from a METAR or from a broadcast such as AWOS. The supplement tells you the frequency of that broadcast. It does not replace the observation.
Compared with the airport data block on the sectional, the supplement is more detailed and more complete. The block and the supplement are not identical, and the supplement is not limited to field elevation. How to read the short block, including tower color and the runway length in hundreds of feet, is on the airport data block page. When the block prints a star for a part-time tower, or *L for lighting limitations, the hours and the limitation text live in the supplement.
How the CTAF is identified
In the Chart Supplement, the common traffic advisory frequency is identified by the letter C inside a circle, printed next to the frequency. That is the mark. The digits next to the circle are the frequency you use. There is no single nationwide CTAF, so the answer is not "it is always 122.9." The frequency itself may be a UNICOM, a MULTICOM, a Flight Service frequency, or a tower frequency, depending on the airport. You read the digits that are printed. You do not substitute a memorized number.
A star in the airport data is the part-time tower mark. It does not identify the CTAF. ATIS is a different broadcast, a recorded terminal information service, and it is labeled as ATIS rather than with the circled C. The CTAF page is where the radio use is explained. This page is how you find the right line in the supplement.
Do not swap the CTAF and the AWOS
AWOS is automated weather. Pilots listen to it to hear the observation. They do not use it to announce their position in the pattern. The CTAF is the frequency for those traffic calls. A supplement entry can list both in one communications paragraph. Read the labels.
A worked entry looks like this. The communications line says CTAF/UNICOM 122.8 with a C in a circle, and AWOS-3 on 119.675. The frequency you monitor to hear traffic self-announcements at that non-towered airport is 122.8, the listed CTAF. The AWOS on 119.675 is the weather broadcast. Choosing the AWOS because it is also a radio frequency, or choosing 122.9 because you remember a common MULTICOM, both miss the circled C that is printed on 122.8. The example is one airport. The next airport may print a different CTAF. The method is the circle, not a favorite frequency.
When you open it
Before a job at a field you do not know, open the Chart Supplement entry and confirm the runway layout, the CTAF, and any remark the sectional block could not hold. Noise-sensitive areas, wildlife notes, lighting limitations, and part-time tower hours are the usual finds. Pair that reading with a current NOTAM check. The supplement is revised on its publication cycle. A runway closure or a frequency change that happened yesterday is in the NOTAM, not necessarily in the last supplement edition you downloaded.
On the knowledge test, a question that names the Chart Supplement, or the older name Airport/Facility Directory, is asking for this detailed airport source. A question that shows a circled C is asking you to call that frequency the CTAF, including when the underlying service is UNICOM. A question that asks whether the supplement adds anything beyond the data block is answered yes: more detail, not a duplicate of the same four numbers.
The trap
Calling the supplement a weather radar, or a certificate record, misses the airport data it actually holds. Calling it identical to the sectional block misses the remarks and the runway layout. Calling every CTAF 122.9, marking it with a star, or announcing on the AWOS misses the C in the circle. The FAA Chart Supplement is the detailed directory. The circled C is the CTAF. AWOS stays the weather frequency.
Use a supplement excerpt on the free practice test. Airport publications are covered in the course.