Hello.
There is an internal table to store known devices .. this is the "PDM".
"legacy" means this is the old format of this table
"opdm" means this is the optimized format allowing more devices before table full and more robustness. This is the recommended FW but this will force to clear PDM (or erase EEPROM) then re-pair ALL devices.
In 3.2x there are two versions in the releas list: Legacy and OPDM , what is the diffrence between them?