This is unique among IMC categories, since it is one of only a few things that a company doesn't have to directly pay money for. It's often generated by other sources of media.

These can be interactive

Sales Promotions $($like contests$)$ or passive $($like coupons$).$

Direct Marketing

We often associate this

$3.$ Public Relations category with spam messages in our email $4.$ Personal Selling inbox.

This is one of the largest categories of IMC related communication in most B$2$B environments. It's less common today in traditional retail business.