What is the reference level on a spectrum analyzer?

For example, on power-up, the reference level is 0 dBm, meaning that the top line on the screen is at 0 dBm and you measure the amplitudes of lines in the spectrum down from that level. For example, if the REF LEVEL is set to +10dBm, a signal peak that reaches the top of the display is +10dBm.

BASICS OF THE SPECTRUM ANALYZER