In some species and some situations, certain signals can be very unambigious. One individual does something or sends out some kind of signal, and that has the same meaning to any other individual that perceives it. When a female moth excretes pheromones, any male with receptors will receive that signal and respond in pretty much the same way to it: moving towards the source. 
However, especially in cognitively complex social animals that form lasting relationships, like parrots, signals can be ambigous. Parrots are not born knowing all the ins and outs of parrot language. This is because parrot communication, just like human language, is learned, to a large extent.


I interact in different ways with my boss, a close friend, my partner, or a total stranger, and I will likely interpret what they say and do differently, based on our relationship, our shared past, with eah other. We are all a part of each other's context, and so we influence what behaviors (including language and body language) we see in others when they are close to us, and vice versa, as we learn what to expect from each other. Parrots also interpret each other differently depending on who they are and their (or lack of) shared past. In other words: they not only look at what the other bird is doing, they also take in to account their previous interactions with that individual, as well as the current context - just like humans do. One person doing something will not always induce the same response in me as another person doing the same thing. Even the same person doing the same thing in two different contexts can induce two very different responses from me.  
As an example, consider the body language we migth see in a sexually mature parrot close to a potential nest site.
We might see very erect feathers, eye pinning, a lowered head, head bobbing, maybe some feaking and chewing. These behaviors can be seen by anyone around the bird; both the parrot's partner and any potential rivals. But what information is it meant to convey, and who is it for? I have no idea what, if any, the intention behind these behaviors are in this context. 
What I do know is that when we see this type of body language in a sexually mature parrot that is investigating a dark and cozy corner, it's very likely that one out of two things will happen if someone approaches the bird: 

a) Ouch. The parrot lunges and probably bites - hard. Defending nest sites is no joke to parrots. 
b) We might see the 
behaviors present continue or intensify, sometimes followed by allopreening and regurgitating food.

Those are some radically different options. What determines which one it is?
Probably several things, but most notably: the relationship the bird has to the individual approaching and their previous experiences with each other. Converesly - whether or not I as a trainer would try to approach a parrot that is displaying that kind of body language in that context depends entirely on the relationship I have to that bird and our previous experiences with each other. There's only one bird I personally would approach in this context, and that is my African Grey female, Eris. Se the video below for an example. 

If a parrot I don't know displays these behaviors I would not put my hand anywhere near it, as it almost certainly would result in a bite. However, Eris and I have known each other for almost 10 years, and I interpret her behavior based on our shared past. In the words of Seyfarth and Cheney (2017) on animal communication: 

What this means is that not even parrots are automatically good at understanding other parrots, just like people aren't always experts at getting each other, either. How we communicate and how our communication is interpreted by the recipient is not just a matter of what I do - it's a matter of in what context I do it in, including who it is directed towards and our past experiences with each other. 

"Meaning emerges from the integration of the signal and the social context." 


Seyfarth & Cheney, The origin of meaning in animal communication
Animal behavior 2017, 127; 339-346