Hey Quick! Around here it depends on who says it. JMO We use it to mean removal of any "ladder" fuel small saplings, privey hedge, invasive shrub like plants and anything else that when lit on fire, provides a step up into the canopy of mature keeper trees. This keeps the fire low and fast along the forrest floor. No real chance of a controled burn getting "uncontroled". Also ment to refer to the available "fuel" on the forrest floor. Too thick and your fire gets too hot in one or more places.
Hope this helps. As far as regs that is usually up to the governing "paying" authority.