Different Metadata Levels: The <proj-id> element may be used directly inside <std-meta> or within <doc-ident> (which is inside the three ISO-related standards metadata elements: <iso-meta>, <reg-meta>, and <nat-meta>).

ISO Note: For ISO-related Processing, this element holds an ISO or ISO associate’s project number. In ISO STS, from which this Tag Set was developed, a <proj-id> element was required in the metadata, but the ISO STS documentation states that authors/adopters would not always be able to provide this identifier. Therefore, software that processes the ISO-specific STS vocabulary (in both NISO STS and ISO STS) needs to be prepared for the empty string as content of this element.