I don't have the full context, but I think you are using

(1-ar(L)) (y_t - m - x_t beta) = (1+ ma(L)) u_t
=>
y_t = ar(L) y_t  + (1-ar(1)) * m +  (1-ar(L)) * x_t * beta  +  (1+ ma(L)) u_t
where m is the constant.

In this case, (1-ar(L)) * x_t would be a lag polynomial in current and past x