In the Purpose of Counseling block, write the reason for the counseling. It can be a single word or a paragraph that fills the entire block.
The key for what is acceptable is communication. If the reader can understand the purpose of the counseling by reading just one word, then one word is sufficient. Otherwise, use as many words as necessary to get your point across.
The form instructs the writer to preface the purpose of counseling with the category of counseling:
Performance/Professional counseling is that type of routine counseling that is delivered periodically such as monthly or quarterly counseling.
Event-Oriented counseling is that type of counseling that is delivered in response to an event. It can be delivered after an event such as writing up a Soldier's failure to report or before an event such as when preparing a Soldier for school.
If the purpose of the counseling neatly fits one category, then assign the appropriate category like this:
Event-Oriented: Failure to Report
Most units don't require this level of rigidity and you can leave out the category of counseling. The goal of effective counseling is communication not classification.
Some people like to include a regulation or UCMJ reference in this block, especially for negative counseling staments but it's not required. If you would like to include a reference that pertains to the subject at hand, check this list of regulations.
Check this page of Event-Oriented counseling statements for examples of Purpose of Counseling entries for event-oriented counseling.
Check this page of Performance/Professional counseling statements for examples of Purpose of Counseling entries for Performance/Professional counseling.