Hello R and thanks again for your post.
The instrument number is the recorder's method of identifying a document. For example, a property deed is normally recorded and is assigned an instrument number (a number stamped in the right hand corner of the document) that provides for a method of identification. Similarly, liens and default notices, which are also recorded in California, are assigned instrument numbers. In short, the "instrument number" is the county recorder's method to identify a recorded or publicly available document.
Here in California, we typically identify properties by their assessor's parcel number or APN, which is the tax assessor's method of identifying a property. When we (as Realtors) speak to the title company, we look for the APN to request preliminary title reports and other information needed to assess the value and condition of a property.
You can use both the APN to find the property and the instrument numbers to find specific documents recorded against the property. Hope this helps!
Grace Morioka, SRES, e-Pro
Area Pro Realty