The essential requirement for different businesses to qualify as being related is that

The answer is their value chains possess competitively valuable cross-business relationships. For different businesses to be related they need to create value for each other.

An example of this would be a company that manufactures chocolate also has a subsidiary business where they sell ready-to-eat chocolate-based drinks. This means that both businesses create value for each other, where one is the provider of raw material while the other is the customer who buys them.