On our fleet of NGs it opens up in whatever mode is commanded in CLB/CRZ/DESC pages.
e.g. If climbing KIAS before conversion to calculated Mach it opens in KIAS, descending from cruise level opens in Mach, unless the KIAS calculated for initial descent is equal to or lower than descent Mach number.
At lower (e.g. FL250) cruise levels, it always defers to KIAS
As described above in our FCOM 2:
IAS/MACH display and airspeed cursors display target speed.