1. a notable success or achievement
that promotion would be another notch in your belt
This expression is not usually used when referring to one, singular success or achievement. The idea of notches on a belt indicates a successive line, so, in this context, the success or achievement in question is usually one of a string of others. That is why it is often used in a professional setting. You need to have notches on your belt to have a good CV, for example. In a social context, it only tends to be used in relation to some form of direct competition, such as in sport. It is similar to the phrase 'another string to your bow.'