A question about preparing the PMS for Descent

[Nikita2024]

The FAQ reads:

To start a descend, the operator should follow the next procedure:

Select the desired altitude on the glareshield panel

On the PMS go to the CRUISE page and update the ALT value to this new altitude and press insert

Press VNAV mode and check ATS and TM are engaged too

And at the bottom of the FAQ, an Important Notes for Descent:

To initiate a correctly computed descent profile, you must keep the cruise ALT value unchanged (the value entered on the CLB or CRZ page). The PMS calculates the required descent altitude difference (and therefore the TOD/BOD distances and required VS) based exclusively on the difference between cruise ALT and BOD ALT.

If you manually change the ALT value to match the BOD ALT (or set both to the same altitude), the system will detect zero altitude difference. As a result:

Descent distance becomes 0

The required descent VS becomes invalid

The TOD and BOD predictions will be incorrect or show as zero

Recommendation: Leave the cruise ALT exactly as it was set during climb/cruise planning. Only enter or modify the BOD ALT when you want to start the descent profile. The cruise ALT must remain the original reference altitude for the calculation to work correctly.

Has anyone made sense of this yet with respect to changing ALT on the CRUISE page? Thanks!