Therefore, they enlisted the services of someone who knew the territory—Sacajewea—to serve as their guide for much of their journey.

By virtue of their position, writers serve as a guides and counselors to their readers (read as "target audience"). The best guides:

• Know the territory (read as "topic")
• Know the social values accepted by those they guide, and
• Know the language (and discourse conventions) used by those being guided.