As I understand it, the checkout and checkin notices use 'repetition markers' to generate a list of item/title information to display in the notice.
The list of fields in the notice editor includes <<items.content>> as an option. If this is not supported it should be removed.
For testing I used following paragraph in my notice templates:
date due <<old_issues.date_due>>
No information was output between ITEMSCONTENT and ITEMS.
items.content only works with overdue and advanced notices.
I think it is not functional, it should ideally not appear in the list on the left side for markers to use in the notice - but not sure this can be done, as the notices are all in "circulation".
(In reply to Jonathan Druart from comment #1)
> items.content only works with overdue and advanced notices.
This is exactly right, and furthermore, items.content relies on the 'itemscontent' option of those scripts if any modifications are to be made to what <<items.content>> displays, so it doesn't make sense to use it anywhere else.
At this point, http://wiki.koha-community.org/wiki/Customising_Notices_and_Slips is the best documentation that we have for the expansion of variables.