Basically, principle components of a multivariable group

can be supposed as the condensed information of the group.

Such that, if you can explain the meaning of group, you

certainly can use it as regressor of other variable.

For example, if you have a consumption function. The equity

market factor is one of the factor which affects consumption,

according to your theory. You can use the principle components

of multivariate group as the regressor to present the factors

of the equity market.