When VFR, pilots need not contact TRACON prior to entry or while in any TRSA, however it is recommended they do so. There are no specific equipment requirements to operate VFR in a TRSA. If pilots ask for and receive radar services from TRACON, they must comply with heading and altitudes assigned or cancel the service. Temporary flight restrictions are designated by NOTAM and are used to clear the airspace in special circumstances that could be hazardous to aircraft not participating in the event for which the TFR was issued. Situations in which TFRs are typically used include forest fires and other natural disasters, air shows, some instances of criminal activity, extensions of restricted airspace to allow expansion of military training operations, and during movement of the President and certain other high-level government officials. A Military Training Route is a specific route allowing high speed, low-level flight by military aircraft for training purposes. Specifically, these routes allow participating military aircraft to exceed the normal 250 knot speed limit which applies to all aircraft operating below 10,000 feet MSL. MTRs are depicted on VFR sectional charts and IFR low-altitude en route charts to assist pilots in locating and avoiding them, but nonparticipating pilots are not restricted from flying within an MTR. While the altitude limits of each route are not depicted on ordinary charts, the numbering system partially reveals this information. Routes are first designated as either VFR (VR) or IFR (IR) routes. Then, a numeric code identifies the individual route. A route with a 4-digit code contains no segment higher than 1,500 feet AGL, while a 3-digit code indicates that one or more segments of the route exist at altitudes higher than 1,500 feet AGL. For example, the route VR1234 would indicate a VFR route exclusively below 1,500 feet AGL, while IR567 would indicate an IFR route with at least one segment higher than 1,500 feet AGL. In order to help alert aircraft to the presence of parachute jumping operations, the FAA maintains a list of designated parachute jump areas in the Airport/Facility Directory. While there is no restriction on operating within a parachute jump area, pilots should exercise extreme caution in such areas. Specific conventions are used to indicate airspace boundaries on VFR sectional and terminal area charts (TACs) for the United States. Class A airspace is not shown on VFR charts, since it is assumed to extend from 18,000 FT to FL600 everywhere.