Nonlinear ARX Models - linear network

Hi,

In matlab, 'The nonlinear ARX structure models dynamic systems using
a parallel
combination of nonlinear and linear blocks, ...'

When I built a nlarx model I could not get the linear part separated ?
I did look in Matlab help, but could get any.

Can please any one who has some insight into this please respond

Regards,
Suren
.