sept-primary is essentially a stand-in for Nintendo's package1ldr, on 7.0.0+. To use it, the caller (normally fusée-secondary) loads the sept-primary binary to `0x4003F000`, loads the 7.0.0+ TSEC firmware to `0x40010F00`, and loads a signed, encrypted payload to `0x40016FE0`.
It is normally stored encrypted/signed. Therefore, if one wishes to build sept-secondary instead of using release builds, one must bring their own keys.