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The Issuer type (basic:AttributeIssuerString prior to V3.4) is a PolicyRule which returns true if the name (generally the SAML entityID) of the system issuing the attributes (usually the IdP itself) matches a supplied string. It's not commonly needed but is of use in "multi-homing" scenarios in which the IdP may be representing multiple sources of attributes.
Schema Type and Location
The The Issuer type itself is defined within in the urn:mace:shibboleth:2.0:afp namespace, the schema for which can be located at http://shibboleth.net/schema/idp/shibboleth-afp.xsd
A deprecated version of this The deprecated basic:AttributeIssuerString type is defined within in the urn:mace:shibboleth:2.0:afp:mf:basic namespace, the schema for which can be located at http://shibboleth.net/schema/idp/shibboleth-afp-mf-basic.xsd
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