Sticking to what you know

 Core businesses

Warren Buffett likes to invest in companies where management focuses on
activities that are within the expertise of the company and not wander off and
spend shareholders’ money in going into areas that they know little about.

Keeping a company on track is obviously an attribute of sound company management
and is a sound investment principle.

Understanding the business

This is really just an extension of Warren Buffett’s investment principle
that one should not invest in a company whose business one does not understand.
If it applies to direct investment, it also applies to indirect investment and
an investor is better off investing in a company that uses its capital in areas
of its own expertise.