AllowCreate in NameIDPolicy element

Hello!

I'm trying to understand the AllowCreate attribute of a
samlp:NameIDPolicy element. SAMLcore states 'When "false", the requester
constrains the identity provider to only issue an assertion to it if an
acceptable identifier for the principal has already been established.'

On an operational level, does this mean that computed or transient
NameIDs cannot be used unless AllowCreate is true?

On the policy level, why should an SP care?

Thanks for any insight,
Alex

-- 
Alex Stuart
Team Leader - Federated Access Management
EDINA, University of Edinburgh

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.