Thank you for your inquiry. ANSI and ISO are organizations that set standards, generally for product quality and
engineering applications. The number represents a specific standard that a paper meets. For example, the ISO 9706 standard deals with paper's resistance to ageing. A summary of ISO 9706 is here.
For any ISO or ANSI standard (with numerical reference), a web search of the term will quickly yield what it means and how it applies.
Where possible, we will link to the standard's definition from our website.