As the Android build system also produces files like system.img and boot.img, theses can be installed through the bootloader (either with fastboot or heimdall) without any signatures check.

That can be useful if you just built Replicant and want to test it without having to generate a zip install file.

The resulting system.img might be a sparse ext4 image which needs to be converted to a normal image before flashing.

Also make sure to refer to the Replicant installation guide to make sure that you are using heimdall or fastboot correctly, to not break your device.