Philips V24CT User's Reference Manual page 255

Viewing Patient Data
Inserting a The Mark Event function also enables you to insert a calibration
Calibration
reference into displayed waves.
Mark
When checking the magnitude of a displayed wave, it can be useful to
have a reference or standard to compare against. The Cal Signals key
momentarily interrupts the displayed ECG, RESP, and pressure waves
and inserts the cal pulse(s) for reference. (This calibration wave is in
addition to the calibration reference bar that is always displayed on the
ECG and RESP waves.)
Since these calibration pulses momentarily become the waveform signal,
they are also communicated over the Philips monitoring network (SDN)
and appear on system displays and recorders. The RS232 and analog
output interfaces also carry these pulses.
If a recording is in progress, the calibration wave also appears on the
recording (in addition to the recorder calibration wave at the start of the
recording).
Trends and Calculations 8-17