Question

If we say that outlines should make use of subordination, we mean that __________. headings should be general, while subheadings should be more specific subheadings are less important than any other part of the outline both headings and subheadings should be complicated headings are unnecessary to most outline structures

Answer 1

If we say that outline should make use of subordination, we mean that HEADINGS SHOULD BE GENERAL WHILE SUBHEADINGS SHOULD BE MORE SPECIFIC. 
In the writing process, outline helps a writer to logically and systematically arrange his materials. Each outline is expected to observe four basic principles, which are: parallelism, coordination, subordination and division. The principle of subordination requires that, In order to indicate the level of significance, an outline should use a principal and subordinate headings. Thus, materials should be ordered from general to specific. 

Answer 2

If we say that outlines should make use of subordination, we mean that headings should be general, while subheadings should be more specific. Subordination elaborates the topic more because of how the headings and subheadings are outlined.