The eigenstate function can give you the eigenstate of a projection of the full Hamiltonian, i.e. of a finite system.
The transmission eigenstate is a scattering eigenstate of the infinite Hamiltonian. The scattering eigenstate has specific boundary conditions, i.e. scattering left or right boundary conditions which means that it will match bloch states going left or right in the electrodes. The transmission eigenstate is the linear combination of scattering eigenstates, which diagonalizes the transmission matrix.
For further details also check the manual.
We will also publish a manual soon, which shows how to resolve the transmission eigenstate in MPSH states