FAQ - various information:

this line is a tuning aid. This makes it very easy to find the exact frequency. In the spectrum display there is a red line at 100 Hz. You bring the 100 Hz peak to this line and the modem will synchronize immediately. A drift “after” locking is not a problem, it is not necessary to always follow the line.

The received, yellow 100 Hz peak should lie reasonably on the red 100 Hz line.

This is not necessary. Once the modem is locked in, it has a relatively large capture range.

Transceiver: a careful frequency correction is possible

SDR console: The frequency must NOT be changed during reception. The SDR console makes tiny dropouts during frequency changes, which lead to reception errors.