It means you need to read Section 28 of the Real Estate Purchase Addendum. Since it's referenced in the listing, it should be available with the seller's disclosures. It could mean very little, but it could have a major impact on the terms of sale.
There is no form in California called a "Real Estate Purchase Addendum" - at least there is no such standard California Association of REALTORS(R) (CAR) form with that name. Lenore may be referring to a set of forms used in the Riverside County area that I'm not familiar with, as there are several areas in the state where local REALTOR(R) associations have developed a local set of forms for standard transactions (here in Silicon Valley we use both CAR forms or our local PRDS forms, whichever supports the specific transaction best).
The CAR form Lenore may be referring to is CAR Form ADM, which is simply titled "Addendum." It has no section numbers on it, though I always add numbers to each entry when I use it.
I recommend you ask the listing agent for a copy of the document they reference in the listing and work with a local agent, like Lenore, who knows the local forms and processes.