The following is described in 11.3.3 of the V850E/MA1 Hardware User's Manual (U14359EJ4V0UM00 (4th edition)):
(3) Serial I/O shift registers (SIO0 to SIO2) and
(4) Reception-only serial I/O shift registers (SIOE0 to SIOE2)
In the explanation above, the operation when TRMDn = 0 for the CSIMn register is described but no description is made for when TRMDn = 1.
Is it possible to receive data when TRMDn = 1 ?
To start communication when TRMDn = 1, you have to write to SOTBn.
Reading SIOn does not start communication.
On the other hand, when TRMDn = 0, reading SIOn will start communication.
Therefore, SIOEn is prepared for when you want to read data without starting communication.
That is, when TRMDn = 1, SIOEn is not needed. That is why it is not described.