In this use case a User Picker (multiple users) custom field will be updated with one or several users depending on the labels contained in the Labels field.
In the popup, choose the value Set field value manually (parser expression)
Expression
Set it to Advanced text and use the following expression
" + " + toString(filterByPredicate(distinct([
"AA" in %{00080} ? "user.one" : null,
"BB" in %{00080} ? "user.one" : null,
"CC" in %{00080} ? "user.two" : null,
"DD" in %{00080} ? "user.two" : null,
"EE" in %{00080} ? "user.three" : null,
"FF" in %{00080} ? "user.three" : null
]), ^% != null))
The duplicated characters must be replaced with your labels and the different users (user.one, user.two, user.three), with the usernames of the users of your choice.
This expression will check if there are some labels in the issue. If there are, it matches the existing labels with one or several predefined users and removes the duplicated users and the empty values in the resulting list.