"This map represents solar PV panel installations registered on the Renewable Energy Certificate Registry from 2001 to 2015"
This registry is not unlike a share trading register. You acquire "shares" by installing systems which - in essence - reduce the demand on the grid, such as solar hot water systems, heat pump hot water systems ... and solar electricity generating systems. How many "shares" depends on how much energy the system produces or displaces. Once acquired, these "shares" can be bought, sold or traded.
There are some rules about how you can get these Renewable Energy Certificates, such as using components from an approved list and having an approved designer design it as well as the installation being compliant with the relevant standards. As a result, it is conceivable that you may have a fully functional installation that uses all the right equipment and is compliant to the safety requirements, but wan't "designed" by an approved person ... and you won't be able to claim any RECs.
As such, those won't appear on that map - but considering these RECs are worth real money, anyone putting in an installation is going to make the effort to get them.