Clarity needed for time horizon change

Not sure if every problem will specify this, but I am unsure about what constitutes a time horizon change. Clearly, the period that you save for retirement, and then actually retire, constitute a change. I understand this, however, the period when you save for a child’s education has also been mentioned as a distinct time horizon, which has led me to more questions. Would saving a substantial sum of money to start a business contitute a time horizon change, or what about saving for overall family expenses before you even have children? Maybe a few other examples someone could list could help clarify, or if there is a more concrete definition someone has, which could be applied easily to various scenarios, would help. Thanks in advance.