The Zone Field is used to pick a Zone from a drop-down menu. Depending on you configuration there may or may not be different zones available for selection in different projects.
Zones User Picker
The Zones User Picker is a selectbox, which lets you select from users that are member of the currently selected Zone Field.
There is a Multi User Picker, which allows selection of multiple users.
Zones Autocompletion User Picker
The Zones Autocompletion User Picker works the same as the Zones User Picker, but is suitable for larger amounts of users.