The Assets series runs the task's actions for every asset that is included by the filter query. It allows dynamically selecting assets, e.g. by date or other volatile information. The filter query uses the same syntax as the global search in the inventory. That means that you can construct the required filter query in the inventory and copy it here for convenience.
The following are available for configuration:
To search for any asset that has a custom field mydate with a date set within the next month, use:
mydate:>0d mydate:<-1m