CTAF is the frequency pilots use at an airport without an operating control tower to self-announce position and intentions so traffic can coordinate without ATC control instructions. AIM 4-1-9 is the description, and the frequency is the one printed for that airport, including the circled C on the sectional. A remote pilot listens to that picture in order to stay clear of the pattern.
The rule
CTAF is not a recorded broadcast of weather and the active runway. That description belongs to ATIS and to the automated weather broadcasts covered on ATIS, AWOS, and ASOS. CTAF is not the frequency ATC uses to issue landing clearances at non-towered airports. There is no tower operating, so there is no landing clearance on that frequency. It is also not an emergency-only frequency.
On a sectional airport data block, the CTAF is the frequency marked with a circled C. ACS chart reading says the same thing: a C inside a circle marks the common traffic advisory frequency. The digits printed beside the symbol are the frequency. The symbol does not replace those digits with 121.5, and it does not mean the ATIS of the nearest Class B airport. Field elevation in the data block is an altitude in feet MSL. Parentheses on a sectional are for obstruction heights in feet AGL, not for field elevation. The Maximum Elevation Figure printed in the quadrant is a terrain figure. Neither one is the CTAF. A tower frequency is not necessarily the CTAF. Read the specific airport entry, and use the current Chart Supplement when you need the full communications line. The Chart Supplement marks the CTAF with a C in a circle as well. The frequency beside it may be a UNICOM, a MULTICOM, a Flight Service frequency, or a tower frequency used as CTAF when the tower is closed.
One case does use a fixed frequency. AIM 4-1-9 says that where an airport has no tower, no Flight Service Station, and no UNICOM, pilots self-announce on MULTICOM, 122.9 MHz. That is the assigned answer for that empty communications line. 122.75 MHz is an air-to-air frequency. 123.45 MHz is not the MULTICOM frequency. 121.5 MHz is an emergency frequency. Do not treat 122.9 as the CTAF of every airport that already publishes a different circled-C frequency.
How to apply it
Find the airport data block and read the circled C before you fly near the field. Write that frequency down with the runway you expect to hear. Listen for position and intention calls: who is inbound, who is departing, which runway they name. You are building a picture of manned traffic so you can comply with §107.43 and stay out of the pattern. The airport data block page is where the rest of that block, including elevation and runway length, is decoded. This page is only the frequency.
If the data block and the Chart Supplement show no tower, no Flight Service, and no UNICOM, the self-announce frequency for that strip is MULTICOM 122.9. If they show a UNICOM or another frequency with the circled C, use the printed frequency. UNICOM can also be asked for advisory information such as wind and a recommended runway. It does not issue a landing clearance or a Part 107 airspace authorization. Phraseology for reading the calls you hear belongs on aviation radio phraseology. This page does not invent a script.
A circled C and a strip with no radio service
The sectional data block shows 122.8 with a C in a circle. That 122.8 is the CTAF. You monitor 122.8, not 121.5, and not the field elevation printed in the data block. You hear pilots state where they are and what they intend to do. You do not hear a controller issue a landing clearance, and you do not hear a continuous weather loop with a phonetic information letter. If a tower frequency is also printed, you do not assume it is the CTAF. The circled C is the mark.
At a different airstrip the Chart Supplement lists no control tower, no Flight Service Station, and no UNICOM. Pilots self-announce on 122.9 MHz. Choosing 122.75 or 123.45 puts you on the wrong channel for that airport. Choosing 122.9 at a field that already has a circled C on 122.8 does the same thing in reverse. Read the airport you are actually near.
The trap
The purpose item calls CTAF a recorded weather broadcast or a frequency where ATC issues landing clearances. AIM 4-1-9 assigns it to self-announce position and intentions where no tower is operating.
The frequency item says every airport uses 122.9, or it hides the CTAF in the elevation or the Maximum Elevation Figure. The circled C identifies the CTAF, and the digits next to it are the frequency to use. 122.9 is MULTICOM for the airport that has no tower, no Flight Service, and no UNICOM. The distractors on that item are 123.45 and 122.75.
The chart item that describes a C in a circle as ATIS, or as 121.5 no matter what digits are printed, misses the symbol. The circled C marks the CTAF. The printed frequency stands.
Listen to the structure of a few calls on the free practice test. The course teaches the circled C and the 122.9 case as two different lookups.