Designing physical interfaces can be complicated, in that there are many factors to consider, but it is not really "complex" in the sense that the rules are fairly well known. Iconography on physical interfaces should generally be a "no brainer". The Neilson Norman Group gives 3 classifications of icons:
1. Resemblance Icons look like what they are representing. These icons are dependent on familiarity with the action they represent.
2. Reference Icons evoke a specific analogy to illuminate the concept. These require not only familiarity with the action, but also with a specific mental model of the action and knowledge of how the analogy used is related to the action
3. Arbitrary Icons simply rely on a shared agreement about the icon representing a specific icon. It is of no use to those who haven't seen it before, but it allows for a looser mental model of the underlying action.
This brings me to one of my least favorite icons that are ubiquitous and unintelligible: the set of power symbol icons. The only one which is generally well known is the "toggle" button (broken circle with a line). The discrete state icons ("0" for off, "|" for on) are for most people meaningless. Looking into it, they are in fact supposed to resemble on and off in binary. In general, this is such a bad metaphor for most people because they have no experience with binary. Depending on your mental model, this icon is either the reference type (if you understand binary) or the arbitrary type (if you just memorize what the symbols mean without understanding the underlying reasoning).
In addition to understanding the types of icons, it is important to guard against your symbol being categorized incorrectly because of the context. The instance I found recently in the back seat of an BMW is a case of mistaken resemblance. The bar and circle are used in a very loose analogy to "air vent on" and "air vent off" however their physical resemblance is easy to mistake with a more direct icon type, resembling "air vent closed" and "air vent open".
There are several mistakes here:
1. If possible, never use the "power symbol" iconography. It is confusing, requiring a mental model of binary digits or of mastering arbitrary icons.
2. "On/Off" is a broken metaphor. The on/off symbols would be appropriate, perhaps, for turning the air conditioning on/off, but the vent does not do this, it simply closes the port to not allow air flow through. The "on/off" metaphor is inappropriate.
3. Always prefer resemblance icons. The photo below is from an Audi vent system, and uses bars of increasing size to represent the changing amplitude of air. This is close to an analogy, because it makes no explicit reference to air (as some cars do with squiggly lines), but it is generally clear. It would be inappropriate to use "amplitude" as an analogy for the right-side control which controls temperature, since you do not think about controlling the amplitude of the heater / AC (although that is generally what you are doing).