User Selector

User Selector	associates the item field to a user by login name.
Auto-Assign	default	an open selection from the whole list of usernames. N.B. do not use this feature if the user list is private.
	1	assign item to its creator - This option is necessary if you want to limit the modification of the item to its creator and administrators.
	2	assign item to "last modified by" - the is filled up by the item creator or editor user name.
Item Owner	default	Is this field an ItemOwner field that determines permissions of the item? (since Tiki17 )
	0	No	tiki >= 17
	1	Yes	tiki >= 17
Email Notification	default	a notification email is sent to the specified user each time the tracker item is modified(or created)
	0	No
	1	Yes	tiki >= 1.10
	2	Only when other users modify the item	tiki >= ... (6?)
Multiple selection	default	Allow selection of multiple users from the list.
	0	No
	1	Yes (complete list)	tiki >= 17
	2	Yes (filterable by group)	tiki >= 17
Group IDs	default: 0	Limit the list of users to members of specific groups. Separate multiple with "\|"
Groups that can modify autoassigned values	default: 0	List of group IDs who can change this field, even without tracker_admin permission. Separate multiple with "\|"
Show real name if possible	default: no	Show real name if possible

The full list of options in Tiki17

For instance a field with options "1" will assign the user who created the tracker item to the field
A field with option "0,1" will let the user to choose a user and to send a mail each time the item is modified (can be convenient for a "assign to field" )
The notification email contains the list of fields with the old and new values for each field that changed.

Display limitation:
tiki >= 1.10
If you want only display some users for a user selector field with no automatic feeding, you can checked the list you want while defining the field.

Note on how to get auto-assignment work in the TRACKER Plugin: In case it did not work as you expected, you could
(1) try to set view="user" in the plugin
(2) hide the field for everyone except for admin, and add it to the plugin's fields parameter. This works for both the group and the user selector fields. One current drawback (in Tiki5.0B1): the field labels are shown in both cases. As long as it is not fixed, one could use pretty trackers to circumvent this problem. Find more details in a related bug tracker item.

Select multiple users filterable by groups

Since Tiki 17, the user selector tracker field alows selecting multiple users, and that selection can be on a full list of site users, of filterable by groups of users:

This is also useful for notifications to user creators/owners of a tracker item, to handle permissions per item when no full permissions are granted in the whole tracker level but only to the items that the user is set as creator/owner of the item.