in the context of 3DS kickass homebrew, a primary exploit is one that allows access to kickass homebrew (homebrew launcher) and requires no previous access to kickass homebrew
for example, soundhax is a primary exploit because you can enter homebrew launcher just by putting the exploited .m4a file and boot.3dsx on your SD card then playing the .m4a file in stock Nintendo 3DS Sound app
secondary exploits grant access to homebrew launcher but require previous kickass homebrew access to install/set up exploited data
menuhax, for example, had an installer that can only be run from homebrew launcher. the installer would put exploit code in the Home Menu extdata that would cause the 3DS to find and execute boot.3dsx instead of loading the Home Menu as normal. in the Dark Ages, you would typically put your CFW loader at boot.3dsx using this method
Safehax would not be classified as either a primary exploit or secondary exploit. its purpose is not to enable kickass homebrew via the homebrew launcher.