Selection list search

Summary

In Fluid, we can type in any value to quickly find it in a long selection list. However, it's not available to all fields. Why?

Content

In Fluid, we can type in any value in a field search block to quickly find it in a long selection list. However, it's not available to all fields with selection list. See below screenshot. Does anyone know what controls this feature? Is the feature only available when the selection list options reach some certain number?