DR = HO from sdcch to tch, detected with a standard HO algorithm (could be poor quality, low level, long distance, power budget, etc)
FDR = ho from sdcch to tch, detected thanks to a specific “capture-style” algorithm:
if DL_RXLEV(n) > L_RXLEV_NCELL_DR(n) and number of free TCH in n > FREELEVEL_DR(n), then the MS is sent to “n”.
The order of detection… DR is actually a mix of different causes, so if DR is detected on Emergency algo, it has high priority. If it is detected with PBGT algo, then it has the same priority as the cause 12.
FDR and fast traffic HO have less priority than cause 12. But the priority is no really important here : DR doesn’t happen much, while FDR and FTH are more easily triggered.