To ensure that the pitch reference in your device is accurate, you should calibrate TuneLab at least once after it is installed.  Thereafter TuneLab will use the results of that calibration to adjust for any inaccuracy in the sound system of your device.  Calibration is not for tuning to a non-standard pitch, such as A-415.  Use the offset on the main tuning screen for that instead.

If you select "Internet Calibration", you must leave your device running the calibration routine for 6 hours. Follow the prompts on the screen after selecting this option. This is best done overnight with your device on a charger. If this process is interrupted before the 6 hours are up, the calibration will be aborted and you will have to start it over from the beginning. Internet Calibration accesses Internet time servers only very briefly at the beginning and again at the end of the 6 hour period. See the manual for more details on how Internet Calibration works.

For any of the other choices for a calibration source, you will need a reference tone with a trusted pitch to use as the reference.  This tone can come from a good tuning fork, although better results can be obtained by using a good electronically-generated tone.  On this page you select the pitch you will be using.  Three common pitch sources are offered as choices.  If you want to use some other frequency, there is a choice that lets you enter any frequency explicitly. Do not use a tone from a website or from an uncalibrated program. Your iPhone's default calibration is already better than that even before you do a custom calibration.

The NIST tones referred to in the first choice are the tones available from the National Institute of Standards and Technology in Colorado, USA.  The tones are available by telephone by calling NIST at (303)499-7111.  Do not use your iPhone to make this call.  Use a separate phone with a loud enough speaker to be heard by TuneLab.