An expression can be:
The rule is that if a value is not fixed in advance, it is regarded as a constant and not an expression prefix is considered a groovy expression.
Predefined variables can be used in xml:
__date__: Today's date
__time__: Date + time of day
__datetime__: Date + time of day corresponding to the local time zone. Used for fields that define the attribute tz = "true"
__user__: User logged
__this__: The values entered in the view of the current record.
__self__: registered in the database of the current record values