Non-specialists
I believe that they are a secondary type of audience they're usually not who you are writing the document for. It should still be kept in mind that there will be non-specialists reading your published document so it is important to try to appeal to them as well.
For example, if you write a set of new procedures for a company’s technicians, you must also include information that your gatekeepers insist accompany the document, such as legal clauses or business descriptions. You will also need to think about the technicians’ bosses (executives) who need to approve any new procedures implemented at the site.
This provides a pretty good example at what should be thought about when writing. You not only have to write information appealing to your primary audience but also keep in mind other useful details for a secondary audience.
The secondary audience(s) is made up of others who may read or be interested in a document, but who is not the main (primary) reader. In the laboratory example, these groups could be experts who enforce laboratory regulations and safety standards, but also could be non-specialist custodial staff that clean and maintain the lab. Both of these groups may be interested in this document—the experts ensure that safety standards are met, while the non-specialists may need to follow the displayed protocols in an emergency. Additionally, the secondary audience may be someone your primary reader consults if your message, report, proposal, etc. is a request for a specific action. For example, if you are a salesperson making a pitch to a client, that client may need to consult their supervisor before agreeing to your terms.
This shows that the reader doesn't always have to be an expert on what the reading is about. This highlights that both experts and non-specialists can use the same information provided for different purposes.