Units for importance from the varimp function applied to random forest model in caret?

Question

I am being asked what the units are for variable importance using the varimp() function for a caret package random forest model. My best answer based on previous posts is % but is that accurate? Would %MSE be a better interpretation, something else entirely or is it unitless and just showing relative magnitude? The caret documentation does not say and previous posts are not definitive on this.