The id(units).us(Trait)structure for repeated and multivariate models is a default.
ASReml ‘knows’ we are fitting a multivariate model if we have more than one dependent
variable listed on the left hand side of the model statement. Even if we do not define the residual term in the model ASReml will automatically adds it. Again, it is a useful habit to write the residual structures explicitly because the user may need to add starting parameter values in some cases.