This field has support for 'mentioning' users. You can bind a store to this textfield, and use the '@' to trigger a lookup of the store. This will display a picker from which you can choose a matching value. Selecting a value will replace the search text with a special syntax '[~some_selected_value]' that can be used for other purposes in other interfaces.
As with Ext.form.field.ComboBox, this custom field can be configured with remote or local querying, and even paging of the results.
Although you could use this with any data, the primary use case for the Mention field is with a person list. To this end, it has been configured to support thumbnails in the search list. All you have to do is add a few configs:
By default, the "@" is the trigger to start a lookup for a value, and the resulting selection will be wrapped like so for inclusion in the raw field value: [~{valueField}].
If needed, you can adjust this to suit your own formats with the following configs:
There is an example application (Ext JS 6) in "examples" that demonstrates several of the different ways that Mention can be configured.