Always read the official Owners Manuals first
UPDATE: information about Fractal Audio's new FM9 and FullRes IRs has been added
- 1 Tempo menu
- 2 Global or per preset
- 3 Channels
- 4 Tap tempo calculation
- 5 Syncing effects to tempo
- 6 Accuracy of the tempo LED
- 7 Metronome
- 8 Remote control
- 9 Tips and tricks
When tapping the Tempo button on the processor hardware or Tempo switch, the Tempo menu appears on the hardware GUI. It will disappear after a few seconds. Some devices let you specify how long it stays on-screen (Global menu).
On the Axe-Fx III, FM9 and FM3, the Tempo button is also a shortcut to the Controllers menu.
Global or per preset
You can specify in the Tempo menu (Tempo To Use) if you want the set tempo to apply to all presets (Global) or per preset (Preset).
The Controllers block in firmware Ares and later has 4 channels. This means that presets can have 4 different tempos. This allows setting separate tempos per scene.
Tap tempo calculation
The Global menu lets you specify how Tap Tempo works:
Average — averages the tempo to the last 10 taps. Usually a wise choice if you can't get the first taps right, and the default setting
Last Two — uses only the most recent two taps to determine the tempo. Acts faster but also less precise
Tap Tempo needs at least two taps to calculate.
Syncing effects to tempo
You can assign effect parameters such as Time or Rate to a specific tempo, like 1/4 or 1/8 Dotted, instead of specifying the number of milliseconds. That will make the effect depend on the global or preset tempo. Tapping a different tempo will change the rate of the effect.
Important: after assigning a parameter to a tempo, you can no longer adjust the time in the effect manually!
Accuracy of the tempo LED
"The tempo LED is handled by a low-priority thread so it isn't rock solid and the higher the CPU usage the less solid it will be." 
The Tempo menu contains a metronome. Read this: Metronome
Scroll to the Tempo CC in the I/O menu and hit Enter. Click the switch on the footswitch that you want to assign the Tuner to.
Or configure it manually by assigning the Tuner MIDI CC to a switch on a MIDI controller.
The default MIDI CC for the Tuner is 14 on the Axe-Fx II, AX8 and FX8. Keep it like this to avoid issues.
The Axe-Fx III, FM9 and FM3 let you set the Tuner CC manually in the I/O menu.
The MFC-101 MIDI foot controller has a dedicated Tempo command for the Axe-Fx II, when it's running in Axe-Fx Mode.
The FC-6 and FC-12 foot controllers have a dedicated Tempo command.
Read this: MIDI Clock
"It all depends on the accuracy of your DAW. If I set the BPM to 130 and the accuracy is 5ms it will occasionally go to 131 BPM on my system here. If I lower the accuracy to 2ms then it doesn't do that. And this is as would be expected. 130 BPM is 2.167 beats per second. MIDI Clock is 24 messages per beat. So that's 52 messages per second or a message every 19.2 ms. 129 BPM translates to 19.38 messages per second. So your messages need to be accurate down to fractions of a ms." 
Tips and tricks
Tempo and Beats Per Minute (BPM)
The Owner's Manuals lists the Tempo values with the BPM equivalents.
BPM on the Performance Pages
If you add Controllers > Tempo as the first control on a Performance Page on the Axe-Fx III, FM9 or FM3, you'll always see the preset's BPM value when switching presets from your foot controller.
Auto-activate a delay by tapping Tempo
Read this: Delay block
SysEx commands for tempo
Forum member Kamil Kisiel provides this Tempo SysEx Generator for the Axe-Fx III and FM3.
Bleed-through tempo noise
Audible bleed-through of the tempo signal can occur if the MIDI cable is not properly shielded. Use high-quality, properly shielded MIDI cables.