Koha doesn't currently have a single point of truth for documentation of notices and slips, which has lead to a proliferation of markup syntaxes, especially in regard to repeating detail, e.g.

DUEDGST/PREDUEDGST:

   <<items.content>>

ODUE*

   <<items.content>> 

   or <item> </item>

CHECKOUT/CHECKIN/RENEWAL

   

   ----

   Detail here

   ----

These have been documented in https://wiki.koha-community.org/wiki/Customising_Notices_and_Slips#Notice_Markup , but this has gotten out of date as new notices get added in each version of Koha.

Work to be done:

Add Koha/Notices.pod which will document the following for each notice in the Letter table:

* Letter Code

* Source file where the notice is populated

* Logic that triggers the notice

* Detail syntax (if any)

* Available data (tables and substitutions)

* Message transport restrictions

The output from this pod should be used as the basis for the Koha user documentation.

See  Bug 21858  and https://wiki.koha-community.org/wiki/Notice_documentation_rfc