Various studies over the years have examined the relationship between content relevancy and behavior. Almost everyone would agree that content must be relevant. But what is relevance? According to Wikipedia: "Relevance describes how pertinent, connected, or applicable something is to a given matter." A thing is relevant if it serves as a means to a given purpose.
In the context of this discussion, the purpose of content is to positively influence customer or employee behavior, such as increasing purchase frequency, purchase velocity (time to purchase), likelihood to recommend, productivity, etc.
When we ask marketers and others how they measure content relevancy, we often hear, "We base it on response rate." If the response rate meets the target, then we assume the content is relevant; if response doesn't meet the target, we assume it's not relevant.