Question regarding proving work experience for self-employed pharmacists

[Dr Ereny]

This is what I found written on the document checklist:

"Self-employed individuals must provide documentation from 3rd party individual(s) indicating the service provided along with payment details. Self-declared main duties or affidavits are not acceptable evidence of work experience."

I need to know what kind of third party is referred to in the checklist ,and what kind of documents is required? does it mean bills or voitures for instance?

how do I prove what I had worked? (should I find some of my former customers to write a statement.)
                                                                         
   ~Thanks