Sub-themes, Niches, Micro Niches and Phrases:

Let's look first at the those categories that are closest to the theme.  In the first illustration you will see a visual depiction of these categories.  Phrases have the smallest breadth of topic.  As the size of each ring increases, so does the topic size of that category.  Micro niches, followed by Niches and then Sub-themes, each progressively broader in topic, all of which are smaller than the parent theme.

Each of these categories are equally close to the parent theme.  A phrase 100 and a sub-theme 100 are both extremely close to the topic of your parent theme, both are excellent candidates for your website.  However the scope of the keyword will demand that they are used in different ways.  Phrases are generally supporting keywords where as sub-themes are quite often silos.  Phrases will never be recommended as a silo because the scope of the word would not support it.  Sub-themes, conversely can and do get recommended as supporting keywords and expert verbiage.