From the manual
The results are stored in the field oo_.shock_decomposition, which is a three dimensional
array. The first dimension contains the M_.endo_nbr endogenous variables. The second
dimension stores in the first M_.exo_nbr columns the contribution of the respective shocks.
Column M_.exo_nbr+1 stores the contribution of the initial conditions, while column M_.exo_nbr+2 stores the smoothed value of the respective endogenous variable in deviations
from their steady state, i.e. the mean and trends are subtracted. The third dimension stores
the time periods. Both the variables and shocks are stored in the order of declaration, i.e.
M_.endo_names and M_.exo_names, respectively.