In addition to representing something else, every sign represents only whatever it a sign of, some specific thing rather than any other thing.
But why? Why must smoke be a sign of fire and not evolution? Why should a fossil imprint be a sign of an individual species of fish? Why should a red and white striped pole be a sign of a barber shop rather than a candy shop?
In each situation there must be some underlying connection between a sign and what it stands for.
In whatever way a sign is related to what it is a sign for, there is another relation underlying and justifying the relation in question, the foundation relation. Natural signs are signs that have natural foundation relations.
Why is smoke a sign of fire rather than something else? Because smoke, the sign, is the effect of the fire, the thing represented by the sign, and fire is the cause of smoke. A total or partial cause, may sometimes be a sign of its effect. Carpenters at work (final causes) or stacks of lumber and bricks (material causes) or an architect's blueprints (efficient causes) are causes that may be a sign of the construction of an effect such as a building. The sign is a sign for whatever it represents because the two are by nature causally connected.
A fossil imprint is the sign of a species of fish rather than something else because it's appearance is similar to the extinct fish. In this example, the sign is similar because the extinct fish was the cause of the sign, the fossil. A picture reminds me of a loved one because it is a likeness of the loved one, because they really look alike.
So a foundation relation may also be one of natural similarity, either alone or in combination with a causal foundation relation.
Furthermore, similarity may sometimes be a similarity of relations rather than things or properties, in which case the sign is a metaphor. Light sometimes is a sign for truth, as when I say truth is light or truth is like light. There is no real similarity between truth and light themselves, but the relations each bears to something else are similar.
The relation between truth and the knowing mind is like the relation between light and the object it illuminates. A lion might make me think of a king, since the lion is believed to be, to other animals, much like a king is to their subjects. When I see a sunny sky, I may think of a smile, since the sun illuminates the sky as a smile illuminates the face.
Whenever the foundation relation is natural, real, and not made-up or artificially contrived by a mind, the signs in question are natural signs. Causation is the most common foundation relation for natural signs, but similarity and possibly other relations such as propinquity, spatial contiguity, or temporal contiguity also bring about natural signs.
A sign whose foundation relation is artificial is an artificial sign. A red-and-white striped pole might seem to be more similar to peppermint candy than to barbering. But it's a sign of barbering and not something else because our culture in its customs and habits, conventionally associates the two, even though it may have been motivated by some vague natural resemblance such as blood and face cream in shaving. Three balls are a sign for a pawn shop and the word "rain" is a sign for rain itself for the same reason.
When the foundation relation is not natural but imposed by choice, artifice, convention, custom or habit, either by an individual or group, it's an artificial, arbitrary, or conventional sign. And when these signs are specially contrived, I call them symbols.
So signs are either natural or conventional.
Language tools are similar to logical tools. And words are signs. But are language tools natural signs or conventional signs? Maybe they're both. Certain words are onomatopoetic or ideographic. When spoken, words such as "ring", "honk", and "bowwow" are similar in sound to all or part of what they represent, but they are still conventional and arbitrarily constructed. In the same way, Chinese and Egyptian written languages are almost entirely ideographic, the written characters appearing similar to what they represent, more originally than in their developed forms. The Chinese character for "move from place to place" is a picture of a foot. But no matter how much language may be based on original similarities between words and things, it's always arbitrarily designed to perform a certain function. So any sign is connected with what it represents either naturally as a natural sign, or at least partially arbitrarily, as a conventional sign.