Bibliographic records can have a production/release date specified in their MARC group tag 366, subfield b. It's in the format yyyymmdd.
When new items are created on the server and linked to a bibliographic record with 366, subfield b, the holdback period set in the collection is automatically added to the production/release date to create the "holdback date". If you try to issue the item before this date then an alert can be displayed.
For example, say you specify a holdback period of 30 days for a collection and set a production/release date of 20160301 for a bibliographic record in that collection. Create and confirm an order for this record. The holdback date for the items is automatically set to 1 April 2016 — i.e. 31 days after the production/release date 1 March 2016.
To specify the holdback period for a collection:
The item's holdback date will not be set automatically if the calculated date is earlier than the current date.
You can also set a holdback date for a specific item on the Other tab in item maintenance.