Barbecue has many regional variations, based on several factors:the type of meat usedthe sauce or other flavoring added to the meatwhen the flavoring is added during preparationthe role that smoke plays in preparationthe equipment and fuel used to cook the meathow much time is spent cooking the meatAt its most generic, any source of protein may be used, including beef, pork, poultry, and fish. The meat could be ground, as with hamburger, processed into sausage or kebabs, and/or accompanied by vegetables. Sometimes the cut of meat (e.g. brisket or ribs) matters; sometimes the cut is irrelevant. The meat may be marinated or rubbed with spices before cooking, basted with a sauce or oil before and/or during cooking, and/or flavored in numerous ways after being removed from the heat. Occasionally, vegetarian alternatives to meat, such as soyburgers and mushroom caps, are prepared similarly.Typically meat is covered with barbecue sauce. Vinegar-base sauce is typical of Southern United States barbecue, while tomato-based sauce is Western United States style.Many forms of barbecuing involve tough cuts of meat that require hours of cooking over low heat that barely exceeds the boiling point of water. Some forms of barbecue use rapid cooking over high heat, being barely distinguishable from grilled meats to those who would make such a distinction. With high heat barbecuing (often called grilling), the food is placed directly above the flame or other source of heat. With low heat barbecuing, the food is off to the side and almost always under a cover, frequently with added smoke for additional flavor. It is generally agreed among the many regions of North America that indirect heat constitutes "barbecuing," while direct heat is the mark of "grilling." Outside of the US this distinction is rarely observed.Sometimes an open flame is required, with the fuel source irrelevant. In other cases, the fuel source is critical to the end result, as when wood chips from particular kinds of trees are used as fuel.