  A social tagging system allows users to add arbitrary strings, called "tags",
on a shared resource to organize and manage information. The Yule--Simon
process, which has shown the ability to capture the population dynamics of
social tagging behavior, does not handle the mechanism of new vocabulary
creation because it assumes that new vocabulary creation is a Poisson-like
random process. In this research, we focus on the mechanism of vocabulary
creation from the microscopic perspective and discuss whether it also follows
the random process assumed in the Yule--Simon process. To capture the
microscopic mechanism of vocabulary creation, we focus on the relationship
between the number of tags used in the same entry and the local vocabulary
creation rate. We find that the relationship is not the result of a simple
random process, and differs between services. Furthermore, these differences
depend on whether the user's tagging attitudes are private or open. These
results provide the potential for a new index to identify the service's
intrinsic nature.
