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