Calibration tones   
While great care may be taken to set up and operate the mixer and recorder so as to operate them at nominal signal levels, the delay in time between recording and subsequent mixing sessions can mean that equipment settings have been changed in the interval. The equipment might even be different for different stages of the production process so it is important to try to reproduce as closely as possible the original settings so as to retain nominal operating levels and signal quality. 
 
This can be achieved by the use of calibration tones which are pure tones of several different frequencies at specific levels which are recorded at the beginning of the tape. These tones can be used to recalibrate the equipment at the beginning of subsequent recording or mixing sessions and restore optimal signal levels. 
 
In digital systems the basic calibration tone is a 1 kHz tone set to the -10 to -14 VU range. This, of course, assumes that the mixer and digital recorder have been adjusted so their levels agree (any discrepancey or offset can be noted). This tone can be used to reset levels and balance for subsequent sessions.  It may be useful to record several tones of different frequency, 100 Hz, 1kHz, and 10kHz for instance, which can be used to adjust and balance playback. Analogue tapes for optical film transfer may also require pink noise as well. It may also useful to include a short sample of the loudest material as well. In either digital or analogue systems, clear labels on the media and the media case are essential: 

Name, contact information,  
 (digital) 
sample rate, content, timings, 
(analogue) 
noise reduction (if any) tape type, content, timings, wind direction (reel to reel), tones for calibration of eq/bias (optional)  
In analogue systems the basic calibration tone is a 1 kHz tone at 0 VU. This, of course, assumes that the mixer and analogue recorder have been adjusted so their levels agree (any discrepancey or offset can be noted). This tone can be used to reset levels and balance for subsequent sessions. Several tones of different frequency, 100 Hz, 1kHz, and 10kHz for instance, can be recorded at 0 VU and used to adjust and balance playback EQ.  A 15 kHz tone at 0 VU can be recorded to allow azimuth alignment of the recorders heads. 
 
These tones should all be recorded to tape long enough to allow the necessary adjustments to be made without to much annoying rewinding of the tape to replay the tones. About 20-30 seconds for each tone should be sufficient.  If a slow reel to reel tape speed of 7 1/2 inches per second is being used the tones should be recorded at a lower level of -10 VU to avoid tape saturation.