Territorial aggression will have a very similar presentation as protective aggression and can, in fact, be very intense.
True territorial aggression is motivated by an instinct to protect the perceived territory from outsiders. This can be the inside of a house, yard, or even a neighborhood that a dog is walked around frequently.
A key indicator of territorial aggression is when a dog is very aggressive toward outsiders (dog or human) when in these areas but otherwise non-aggressive when in new or neutral areas.
Since territorial aggression will have a similar presentation as protective or fear aggression it can be masked by fear aggression if the dog is also acting in a similar manner off property due to fear. Therefore, a dog that displays aggression on and off property to those that seem to pose little threat may or may not also have a layer of territorial aggression.
Territorial aggression is never directed toward "pack members". Therefore, dogs that protect resting places, crates, food, or rooms in the home from other family members should never be labeled "territorial". Instead, those behaviors need to be correctly identified as dominance or resource guarding. If a dog was territorial aggressive toward a family member, technically the dog would attack the family member as soon as the member entered the yard/home. Different forms of aggression are motivated by different reasons and therefore must be treated differently to manage any problems successfully.
Wild canines are fiercely territorial, and the biggest killer of wolves are other wolves due to conflict over territory. Humans have taken advantage of this instinct during the domestication of many guardian breeds. However, keep in mind that if a wolf shows aggression toward pack mates within their own territory it isn't territorial aggression. The same goes for domestic dogs.
Another behavior correlated and confused with territorial aggression is Alert Barking. Alerting barking is simply the enhancement of the muffled "woof" sound a wild canine makes to warn other pack mates of a possible threat. Technically it is not considered aggressive behavior since, at the core, the motivation behind the bark is to alert a warning to pack members and not directly threaten.