abis allocation

I want your opinion about ABIS ALLOCATION exactly for the parameters fixed or flexible because in ALEX their definitions are:

abisalloc Abis path allocation method
This parameter indicates the allocation method for the Abis paths for a Transceiver Group (TG).
The availability of this parameter depends on commercial agreements.
FIXED
All Abis paths are allocated at TS handler configuration.
FLEXIBLE
Abis paths for E-TCHs and G-TCHs are allocated at call setup and deallocated at call release.

And when we execute the command RXMDP:MOTY=RXOTS for the site which had the abisalloc is fixed I find that the TS of TRX’S are connected at random to the devices of ABIS?

SO I NEED YOUR OPINION TO UNDERSTAND THAT CONFIGURATION