This is the act of depositing published material in six designated libraries and archives. It is a legal requirement that one copy of all material published in the UK must be sent to each of these libraries.
These libraries are (in no particular order):
The British Library
The Bodleian Library, Oxford
The University Library, Cambridge
The National Library of Scotland, Edinburgh
The Library of Trinity College, Dublin
The National Library of Wales, Aberystwyth
As part of the Publishing Essentials kit, we will sort out the printing and deposit of these six copies of your book.