Hi,
From the TRM:
"Bit 0 of the transmission request X register represents byte 0 of the transmission request 1 register. If one or more bits in this byte are set, bit 0 of the transmission request X register will be set.".
I suppose bit 2, 4, 6, ... are for transmission request 2, 3, 4, ... register and so on...
But it is not specified for what bits 1, 3, 5, ... are used.
Best,
Vasili