AtoM is a very permissive system, and will not require you to add unique authority record identifiers if you don't choose to do so. In the upcoming 2.7 release
, AtoM will provide authenticated users with a warning if a non-unique authority record identifier is added in the system (that is, the authority record identifier duplicates another existing authority record identifier in AtoM). However, these are similar to the existing warnings in AtoM related to fields that the relevant standard marks as required - they will only be shown to logged in users, and can be ignored if you choose. For more on this upcoming feature, see:
That said, here's what the ISAAR-CPF standard has to say about forming authority record identifiers:
5.4.1 Authority record identifier
To identify the authority record uniquely within the context in which it will be used.
Record a unique authority record identifier in accordance with local and/or national conventions. If the authority record is to be used internationally, record the country code of the country in which the authority record was created in accordance with the latest version of ISO 3166 Codes for the representation of names of countries.
Where the creator of the authority record is an international organization, give the organizational identifier in place of the country code.
ES47161AGS RA 00001
Spain, Archivo General de Simancas
Note: For the authority record: Consejo de Guerra
United Kingdom, The National Archives: Historical Manuscripts Commission
AU NLA 93-535878
Australia, National Library of Australia
AU NAA CA 37
Australia, National Archives of Australia
If you are copying an authority directly from LC, then I see no reason why you couldn't also use the provided identifier. However, if you are making local customizations, then you may want to list the original LC starting record in the Sources field (ISAAR 5.3.8), and create your own identifier.
Perhaps other users may share how they manage their own local authority record identifiers as well.