A linear style may be hierarchical in that it is defined to have more than one level. The style level is simply the number referring to where a block of text falls in that hierarchy, with 1 being the top level.
An outline, for example, is a hierarchy with potentially unlimited levels, each indicated by further indentation.
The initial style level is always 1. If the style level is increased, the new number is shown on the status line along with the name of the linear style. Changing the style level has NO EFFECT for linear styles that do not have an appropriate definition. An appropriately defined hierarchical linear style will have the [hi] code somewhere in its on codes. The [hi] code is described further in the DBT CODES QUICK REFERENCE Topic.
To change the style level, use Layout: Style Level+ or Layout: Style Level- as appropriate.