just to say, modern materials can't elongate an inch, 2 inches, or anywhere close to that. when that happens, the material isn't elongating. its something else.
To be correct, it's most likely both. The material is both elongating and the slack created by poor construction methods, comes out of the string. Some materials stretch more than others. Eventually stretch will turn into a permanent creep. This is why I have been a fan of Ultracam. It's the material with the highest percentage of Vectran, therefore the least amount of stretch. Combine this with the best construction techniques and you have a string that is extremely stable.
The only warning I give people, is to make sure to get a new string every year, if they get one from me. High Vectran-content strings are more brittle and can break more easily without warning. I've never had one break within a year of construction, even with 30,000 shots on it, but it's better to be safe than sorry.