std-id ISO-specific Standard Identifier (for a citation)
In ISO STS, this attribute named the designator for the standard being cited (in the element <std>). Current NISO STS Best Practice does not use this attribute; rather the identifier for a standard being cited should be tagged either
NISO STS Best Practice
The information that this @std-id attribute conveyed in ISO STS should instead be tagged as element content inside the element <std>, using either a <std-id> element for one designator or a <std-id-group> to record more than one designator. Using an element to hold this designator is preferable because it is frequently necessary to list more than one standard identifier, and an attribute can hold only one. The @std-id attribute has been retained for backwards compatibility.