Good question.
As concepts come into common usage in the culture at large and the signing community has a need to use or discuss these concepts signs emerge among users. Sometimes more than one sign will initially emerge but one may take over as most consistent with the rules of how sign-words are formed in ASL. Usually signers defer in general to person who are considered most native to the language. These are individuals who have used ASL since birth because they have deaf signing parents. There is no "Commission on the Correctness of Sign Language" just as there are generally no governing groups controlling English or other languages. The users of a language control through their common usage and understanding what is "correct" in the language.
As an example of how signs develop within a signing community, years ago when I was a professor at the National Technical Institute for the Deaf at the Rochester Institute of Technology a circumstance developed where there was over capacity in the dorms and the college was for a temporary period putting three beds in rooms that were intended for two students. This was called "tripling up". When, after a period of adjustment the university was able to reduce the number of students back to two per room they used the term "de-tripling". During this period students developed two signs. If their room was being "tripled up" they used a sign with non-dominant palm hand held in front and the dominant hand forming a two with palm down facing their non-dominant hand and then moving the hand up while changing the handshape from a two to a three. This meant "tripling up". When the circumstance developed that their room was being moved back ("detripled") to two students the opposite formation of the sign was used with the dominant hand starting higher with a three handshape and moving down closer to the non-dominant hand changing as it moved to a two handshape. The students were able to talk about this "tripling" or "detripling" with these two sign creations that did not exist prior to this circumstance developing at their school. The signs do not exist today in ASL and were used only among the signers in this limited community of the University campus. But this example shows how these signs can develop. The two signs were accepted among signers because they follow the principles of how sign language creates meaningful sign-words. They utilized several principles of sign formation. Sign language is generally speaking consistent with physics so "up movements" generally mean things are increasing or positive while downward movements are associated with negative or decreasing. These two signs incorporated this up or down movement which is consistent with how other signs represent reality. The signs also incorporated a number handshape "two" and "three" which follow a principle in ASL whereby signs can convey the number of things, people etc by changing the number handshape incorporated into the sign so for example the same basic motion is used for "ONE-WEEK", "TWO-WEEKS" and "THREE-WEEKS" with the only difference being the number handshape changing within the sign to indicate how many weeks. So this "number incorporation" rule was being used in the signs that meant to "TRIPLE" or "DETRIPLE".
Thanks for your good question. Please let me know if you have additional questions.
