What do ALTRV NOTAMs typically describe?

ALTRV NOTAMs, or Alternate Route Notices to Airmen, are specifically designed to inform pilots and relevant personnel about areas of airspace that are established for special use or purposes, typically for operations that deviate from standard flight paths. The primary focus of ALTRV NOTAMs is to outline the area in use and the duration for which these alternate routes are active. This information is critical for ensuring safety and coordination in the airspace, especially during military exercises or when temporary airspace restrictions might be in effect.

In contrast, the other choices involve aspects that are not the main focus of ALTRV NOTAMs. Descriptions of the flight path relate to general navigation and route planning, while weather conditions pertain to atmospheric conditions that can impact flying but are not specified in ALTRV notices. Aircraft types do not fall under the scope of what ALTRV NOTAMs convey; instead, they pertain to the specific capabilities or classifications of the aircraft but are not the subject matter of these NOTAMs. Therefore, the description of the area in use and its duration is what makes the correct answer accurate.